build-system/gnu: If a phase returns #f, the build fails.

Fixes <https://bugs.gnu.org/31974>.
Introduced by commit d8a3b1b9e8.

* guix/build/gnu-build-system.scm (gnu-build): Use 'every' instead
of 'for-each'.

guix/build/gnu-build-system.scm

@@ -792,7 +792,7 @@ in order.  Return #t if all the PHASES succeeded, #f otherwise."
 
   ;; The trick is to #:allow-other-keys everywhere, so that each procedure in
   ;; PHASES can pick the keyword arguments it's interested in.
-  (for-each (match-lambda
+  (every (match-lambda
            ((name . proc)
             (let ((start (current-time time-monotonic)))
               (format #t "starting phase `~a'~%" name)