guix system: reconfigure: Always use "/" as GRUB's target file system.

* guix/scripts/system.scm (perform-action) <reconfigure>: Wrap
  'install-grub' call in 'false-if-exception'.  Always use "/" as the
  target.

guix/scripts/system.scm

@@ -287,7 +287,8 @@ (define* (perform-action action os
             ((reconfigure)
              (mlet %store-monad ((% (switch-to-system os)))
                (when grub?
-                 (unless (install-grub grub.cfg device target)
+                 (unless (false-if-exception
+                          (install-grub grub.cfg device "/"))
                    (leave (_ "failed to install GRUB on device '~a'~%")
                           device)))
                (return #t)))