gexp: Build 'gexp->file' derivations locally.

* guix/gexp.scm (gexp->file): Add #:local-build? argument.

guix/gexp.scm

@@ -377,7 +377,8 @@ (define (gexp->file name exp)
                     (gexp
                      (call-with-output-file (ungexp output)
                        (lambda (port)
-                         (write '(ungexp exp) port))))))
+                         (write '(ungexp exp) port))))
+                    #:local-build? #t))