install: Files in the tarball are all root-owned.

Fixes a thinko introduced in 175ced4.

* gnu/system/install.scm (self-contained-tarball): Use "root:0" as the
  owner and group.
* doc/guix.texi (Binary Installation): Revert 175ced4.

doc/guix.texi

@@ -264,12 +264,6 @@ verify the authenticity of the tarball against it!}, where @var{system}
 is @code{x86_64-linux} for an @code{x86_64} machine already running the
 kernel Linux, and so on.
 
-@item
-Create the group and user accounts for build users as explained below
-(@pxref{Build Environment Setup}).  Use the exact same names as given in
-the example so that files extracted from the archive will have the right
-ownership.
-
 @item
 As @code{root}, run:
 

gnu/system/install.scm

@@ -67,14 +67,10 @@ (define %root "root")
                                              #:closure "profile")
 
           ;; Create the tarball.  Use GNU format so there's no file name
-          ;; length limitation.  Use the owner and group names given in the
-          ;; manual.
-          ;; XXX: /var and /root should rather be root-owned, but it doesn't
-          ;; make any difference in practice.
+          ;; length limitation.
           (with-directory-excursion %root
             (zero? (system* "tar" "--xz" "--format=gnu"
-                            "--owner=guixbuilder01"
-                            "--group=guixbuild"
+                            "--owner=root:0" "--group=root:0"
                             "-cvf" #$output ".")))))
 
     (gexp->derivation "guix-tarball.tar.xz" build