pack: Use guile-for-build for the target system.

Until now, running "guix pack -s i686-linux" on an x86_64-linux machine,
for instance, would use an x86_64 guile for module derivations.  This
was OK until now, but would break when passing "--localstatedir" due to
the introduction of guile-sqlite3: we'd be using the i686 guile-sqlite3
along with the x86_64 guile.

* guix/scripts/pack.scm (guix-pack): Pass the 'system option from OPTS
to 'package-derivation'.

guix/scripts/pack.scm

@@ -722,6 +722,7 @@ Create a bundle of PACKAGE.\n"))
                                         (if (assoc-ref opts 'bootstrap?)
                                             %bootstrap-guile
                                             (canonical-package guile-2.2))
+                                        (assoc-ref opts 'system)
                                         #:graft? (assoc-ref opts 'graft?))))
         (let* ((dry-run?    (assoc-ref opts 'dry-run?))
                (relocatable? (assoc-ref opts 'relocatable?))