After this change, only obs-portal backup will prune the repository of
old snapshots. This is the last backup service to run at 06:00 AM UTC.
This should avoid our nightly backups failing because of the exclusive
lock on the restic repo. We currently start the next backup while the
previous one is still pruning, which makes the newly started one fail
with:
repo already locked, waiting up to 0s for the lock
unable to create lock in backend: repository is already locked by PID 228…
