build-system/gnu: Set 'SOURCE_DATE_EPOCH'.

Suggested by Ludovic Courtès <ludo@gnu.org>.

* guix/build/gnu-build-system.scm (gnu-build): Set SOURCE_DATE_EPOCH for
  deterministic builds.

guix/build/gnu-build-system.scm

@@ -576,6 +576,9 @@ (define (elapsed-time end start)
   ;; Encoding/decoding errors shouldn't be silent.
   (fluid-set! %default-port-conversion-strategy 'error)
 
+  ;; Avoid non-determinism related to generated timestamps.
+  (setenv "SOURCE_DATE_EPOCH" "1")
+
   ;; The trick is to #:allow-other-keys everywhere, so that each procedure in
   ;; PHASES can pick the keyword arguments it's interested in.
   (every (match-lambda