guix system: Always build grub.cfg for 'init' and 'reconfigure'.

Fixes <http://bugs.gnu.org/21068

>.
Reported by Germano Gabbianelli <tyrion.mx@gmail.com>
and Mark H Weaver <mhw@netris.org>.

* guix/scripts/system.scm (perform-action): Always add GRUB.CFG to DRVS
  for 'init' and 'reconfigure'.

Co-authored-by: Mark H Weaver <mhw@netris.org>