Fix check mode for the backup task

The "archive" module will fail the playbook on an empty source list,
even in check mode. Thus, explicitly disable it in check mode.

roles/taiga-back/tasks/backup.yml

@@ -61,6 +61,7 @@
   tags:
     - backup
     - offline
+  when: not ansible_check_mode
 
 - name: fetch backup archive
   become: true