environment: Honor ‘-s’ for guile-for-build.

Partly fixes <https://issues.guix.gnu.org/65225>.

* guix/scripts/environment.scm (guix-environment*): Pass SYSTEM to
‘package-derivation’ when defining ‘%guile-for-build’.

Reported-by: Simon Tournier <zimon.toutoune@gmail.com>
Change-Id: I9d883f1e61fac72754fdc9dee9b7c8a3dea32add

guix/scripts/environment.scm

@@ -1153,7 +1153,8 @@ command-line option processing with 'parse-command-line'."
                               store
                               (if bootstrap?
                                   %bootstrap-guile
-                                  (default-guile))))))
+                                  (default-guile))
+                              system))))
           (run-with-store store
             ;; Containers need a Bourne shell at /bin/sh.
             (mlet* %store-monad ((bash       (environment-bash container?