pull, time-machine: Add '-q' to ignore channel files.

This also fixes <https://issues.guix.gnu.org/63726>.

* guix/scripts/pull.scm (show-help, %options): Add '-q'.
(channel-list): Honor it.
* guix/scripts/time-machine.scm (show-help, %options): Add '-q'.
* doc/guix.texi (Invoking guix pull, Invoking guix time-machine):
Document it.

Reported-by: Simon Tournier <zimon.toutoune@gmail.com>

doc/guix.texi

@@ -4802,9 +4802,11 @@ Specifically, @command{guix pull} downloads code from the @dfn{channels}
 @item
 the @option{--channels} option;
 @item
-the user's @file{~/.config/guix/channels.scm} file;
+the user's @file{~/.config/guix/channels.scm} file, unless @option{-q}
+is passed;
 @item
-the system-wide @file{/etc/guix/channels.scm} file;
+the system-wide @file{/etc/guix/channels.scm} file, unless @option{-q}
+is passed;
 @item
 the built-in default channels specified in the @code{%default-channels}
 variable.
@@ -4906,6 +4908,11 @@ Read the list of channels from @var{file} instead of
 evaluates to a list of channel objects.  @xref{Channels}, for more
 information.
 
+@item --no-channel-files
+@itemx -q
+Inhibit loading of the user and system channel files,
+@file{~/.config/guix/channels.scm} and @file{/etc/guix/channels.scm}.
+
 @cindex channel news
 @item --news
 @itemx -N
@@ -5051,7 +5058,9 @@ guix time-machine --commit=v1.2.0 -- \
   environment -C --ad-hoc guile -- guile
 @end example
 
-The command above fetches Guix@tie{}1.2.0 and runs its @command{guix
+The command above fetches Guix@tie{}1.2.0 (and possibly other channels
+specified by your @file{channels.scm} configuration files---see
+below) and runs its @command{guix
 environment} command to spawn an environment in a container running
 @command{guile} (@command{guix environment} has since been subsumed by
 @command{guix shell}; @pxref{Invoking guix shell}).  It's like driving a
@@ -5061,6 +5070,21 @@ invocation can be expensive: it may have to download or even build a
 large number of packages; the result is cached though and subsequent
 commands targeting the same commit are almost instantaneous.
 
+As for @command{guix pull}, in the absence of any options,
+@command{time-machine} fetches the latest commits of the channels
+specified in @file{~/.config/guix/channels.scm},
+@file{/etc/guix/channels.scm}, or the default channels; the @option{-q}
+option lets you ignore these configuration files.  The command:
+
+@example
+guix time-machine -q -- build hello
+@end example
+
+will thus build the package @code{hello} as defined in the main branch
+of Guix, without any additional channel, which is in general a newer
+revision of Guix than you have installed.  Time travel works in both
+directions!
+
 @quotation Note
 The history of Guix is immutable and @command{guix time-machine}
 provides the exact same software as they are in a specific Guix
@@ -5110,18 +5134,21 @@ string or the name of a tag), or @var{branch}.
 Read the list of channels from @var{file}.  @var{file} must contain
 Scheme code that evaluates to a list of channel objects.
 @xref{Channels} for more information.
-@end table
 
-As for @command{guix pull}, the absence of any options means that the
-latest commit on the master branch will be used.  The command
+@item --no-channel-files
+@itemx -q
+Inhibit loading of the user and system channel files,
+@file{~/.config/guix/channels.scm} and @file{/etc/guix/channels.scm}.
+
+Thus, @command{guix time-machine -q} is equivalent to the following Bash
+command, using the ``process substitution'' syntax (@pxref{Process
+Substitution,,, bash, The GNU Bash Reference Manual}):
 
 @example
-guix time-machine -- build hello
+guix time-machine -C <(echo %default-channels) @dots{}
 @end example
 
-will thus build the package @code{hello} as defined in the master branch,
-which is in general a newer revision of Guix than you have installed.
-Time travel works in both directions!
+@end table
 
 Note that @command{guix time-machine} can trigger builds of channels and
 their dependencies, and these are controlled by the standard build

guix/scripts/pull.scm

@@ -84,6 +84,9 @@ (define (show-help)
 Download and deploy the latest version of Guix.\n"))
   (display (G_ "
   -C, --channels=FILE    deploy the channels defined in FILE"))
+  (display (G_ "
+  -q, --no-channel-files
+                         inhibit loading of user and system 'channels.scm'"))
   (display (G_ "
       --url=URL          download \"guix\" channel from the Git repository at URL"))
   (display (G_ "
@@ -133,6 +136,9 @@ (define %options
   (cons* (option '(#\C "channels") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'channel-file arg result)))
+         (option '(#\q "no-channel-files") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'ignore-channel-files? #t result)))
          (option '(#\l "list-generations") #f #t
                  (lambda (opt name arg result)
                    (cons `(query list-generations ,arg)
@@ -735,6 +741,9 @@ (define (channel-list opts)
   (define file
     (assoc-ref opts 'channel-file))
 
+  (define ignore-channel-files?
+    (assoc-ref opts 'ignore-channel-files?))
+
   (define default-file
     (string-append (config-directory) "/channels.scm"))
 
@@ -750,9 +759,11 @@ (define (load-channels file)
   (define channels
     (cond (file
            (load-channels file))
-          ((file-exists? default-file)
+          ((and (not ignore-channel-files?)
+                (file-exists? default-file))
            (load-channels default-file))
-          ((file-exists? global-file)
+          ((and (not ignore-channel-files?)
+                (file-exists? global-file))
            (load-channels global-file))
           (else
            %default-channels)))

guix/scripts/time-machine.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2019 Konrad Hinsen <konrad.hinsen@fastmail.net>
-;;; Copyright © 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2019, 2020, 2021, 2023 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2021 Simon Tournier <zimon.toutoune@gmail.com>
 ;;; Copyright © 2023 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;;
@@ -62,6 +62,9 @@ (define (show-help)
 Execute COMMAND ARGS... in an older version of Guix.\n"))
   (display (G_ "
   -C, --channels=FILE    deploy the channels defined in FILE"))
+  (display (G_ "
+  -q, --no-channel-files
+                         inhibit loading of user and system 'channels.scm'"))
   (display (G_ "
       --url=URL          use the Git repository at URL"))
   (display (G_ "
@@ -86,6 +89,9 @@ (define %options
   (cons* (option '(#\C "channels") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'channel-file arg result)))
+         (option '(#\q "no-channel-files") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'ignore-channel-files? #t result)))
          (option '("url") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'repository-url arg