Avoid running backup-gitlab twice; reuse tarballs

The official backup tool for GitLab takes many hours to run because it
puts everything inside tarballs and then gzips each one. It seems safe
and much more efficient to skip this step for the offsite backup while
reusing the tarballs generated by the first backup to the Storage Box.

Should save ~5 hours from the borg-backup-offsite.service execution.