ci-package-builder: Use the monitoring runner for the OBS logs

The lightweight runners are quite powerful but we only have a limited set of them. When uploading many packages we can easily consume all the slots for a very long time, impacting other users. This makes appying changes across all projects tricky, since we need to rate limit them rather than going fire and forget.

To avoid that, use a separate set of runners with a much more constrained environment but way more slots: this leads to a more efficient use of resources and avoids impacting other users.

Note that this has been in use in production for a couple of weeks as I took an hackish shortcut to avoid blocking everyone when I created 4000+ release commits. It's now time to land this properly. :D