vellum reenable hooks should be protected from premature mount-restore calls

[rmitchellscott]

If any hook calls mount-restore itself, it will remount the filesystem read-only before subsequent hooks run, causing them to fail.

Should have vellum reenable set an environment variable (VELLUM_REENABLE or similar) and modify mount-restore to skip execution when this variable is set:

  # mount-restore
  if [ "$VELLUM_REENABLE" = "1" ]; then
      echo "Skipping mount-restore (managed by vellum reenable)"
      exit 0
  fi