channels: 'build-from-source' restores '%guile-for-build'.

Not restoring it would cause problems when running:

  guix time-machine --commit=6298c3ffd9654d3231a6f25390b056483e8f407c

or similar because the target Guix would be built with 2.2, and then
we'd erroneously go on and attempt build the profile with 2.2.  This
would fail because profile dependencies such as "guile-gdbm-ffi" now
target 3.0.

* guix/channels.scm (call-with-guile): New procedure.
(with-guile): New macro.
(build-from-source): Use it instead of calling 'set-guile-for-build'
just once.  This ensures that '%guile-for-build' is restored
afterwards.

guix/channels.scm

@@ -347,6 +347,21 @@ (define* (guile-for-source source #:optional (quirks %quirks))
       (((predicate . guile) rest ...)
        (if (predicate source) (guile) (loop rest))))))
 
+(define (call-with-guile guile thunk)
+  (lambda (store)
+    (values (parameterize ((%guile-for-build
+                            (if guile
+                                (package-derivation store guile)
+                                (%guile-for-build))))
+              (run-with-store store (thunk)))
+            store)))
+
+(define-syntax-rule (with-guile guile exp ...)
+  "Set GUILE as the '%guile-for-build' parameter for the dynamic extent of
+EXP, a series of monadic expressions."
+  (call-with-guile guile (lambda ()
+                           (mbegin %store-monad exp ...))))
+
 (define (with-trivial-build-handler mvalue)
   "Run MVALUE, a monadic value, with a \"trivial\" build handler installed
 that unconditionally resumes the continuation."
@@ -385,10 +400,7 @@ (define script
         ;; Note: BUILD can return #f if it does not support %PULL-VERSION.  In
         ;; the future we'll fall back to a previous version of the protocol
         ;; when that happens.
-        (mbegin %store-monad
-          (mwhen guile
-            (set-guile-for-build guile))
-
+        (with-guile guile
           ;; BUILD is usually quite costly.  Install a "trivial" build handler
           ;; so we don't bounce an outer build-accumulator handler that could
           ;; cause us to redo half of the BUILD computation several times just
@@ -750,3 +762,7 @@ (define* (channel-news-for-commit channel new #:optional old)
       (if (= GIT_ENOTFOUND (git-error-code error))
           '()
           (apply throw key error rest)))))
+
+;;; Local Variables:
+;;; eval: (put 'with-guile 'scheme-indent-function 1)
+;;; End: