Flesh out clenup job

[alexlarsson]

I added a cleanup workers job at: https://flathub.org/builds/#/builders/8
It just queues a job (with a per-worker exclusive lock) that runs this script:
https://github.com/flathub/buildbot-config/blob/master/scripts/cleanup.sh

Currently that just runs some command to print how much free diskspace there is and how much space are used by various builds. We should add to this code to clean up old builds, because while old builds can be useful for speeding up re-builds they can take up a lot of diskspace.

The one thing we maybe can't always delete is the downloaded sources, because it may happen that at the time of a later build the network source is gone. However, a shared source archive like in #16 would help with that too.