guix system: Fix type error in 'reconfigure'.

Partly fixes <http://bugs.gnu.org/18082>.
Reported by Mark H Weaver <mhw@netris.org>.

* guix/scripts/system.scm (perform-action) <reconfigure>: Pass the
  output file name of GRUB.CFG to 'install-grub'.

guix/scripts/system.scm

@@ -299,7 +299,8 @@ actions."
              (mlet %store-monad ((% (switch-to-system os)))
                (when grub?
                  (unless (false-if-exception
-                          (install-grub grub.cfg device "/"))
+                          (install-grub (derivation->output-path grub.cfg)
+                                        device "/"))
                    (leave (_ "failed to install GRUB on device '~a'~%")
                           device)))
                (return #t)))