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.

roles/borg_client/templates/borg-backup.service.j2

 [Unit]
 Description=Borg backup
+
+{% if inventory_hostname == "gitlab.archlinux.org" %}
+# The ordering relation defined below is important for the GitLab backups
+# because the offsite backup reuses the tarballs from this service's run.
+{% endif %}
 Wants=borg-backup-offsite.service
 Before=borg-backup-offsite.service
 

roles/borg_client/templates/borg-backup.sh.j2

@@ -45,7 +45,8 @@ if systemctl is-active mysqld || systemctl is-active mariadb; then
     /usr/local/bin/backup-mysql.sh || true
 fi
 
-{% if inventory_hostname == "gitlab.archlinux.org" %}
+{# When backing up to offsite, reuse the existing tarballs from the previous backup #}
+{% if inventory_hostname == "gitlab.archlinux.org" and item['suffix'] != '-offsite' %}
 # Create tarball backups of various GitLab directories using the official backup tool
 systemctl is-active docker && /usr/local/bin/backup-gitlab.sh
 {% endif %}