bt forgejo_actions

[RagnarGrootKoerkamp]

Add bt forgejo_actions that can be run from each contest in a git repo.
The contest can either be the root, or /<contest>.

The docs contain some additional configuration for the runners. Everything else
should just work.

[RagnarGrootKoerkamp]

Currently we always pull the latest version of BAPCtools before running, but this requires /opt/hostedtoolcache to be mounted as a shared cache to be efficient (avoid constantly re-pulling).
We could also just use the version in the ragnargrootkoerkamp/bapctools docker container, which may be not the latest, but for those not working on bapctools that doesn't matter anyway probably.

Maybe we could put using the latest behind a flag: --use-latest-bapctools that's off by default.

[RagnarGrootKoerkamp]

Added a --latest-bt flag so the user can explicitly ask for a recent image to be cached. Otherwise the BAPCtools from the docker is used, which is a bit simpler and doesn't require a cache volume to be mounted.

Only remaining TODOs for later:

• Forgejo notifactions for when CI fails.
• Triggering on pull requests as well.
• Optionally, clean up the dependency on the checkout action by doing a manual clone, like here:
  https://github.com/RagnarGrootKoerkamp/research/blob/master/.github/workflows/push.yml#L17