Integrate automatic OBS workflow trigger

[weltling]

This would be useful for both PRs and pushes. The support Github/Gitlab integration has been announced and documented:

https://openbuildservice.org/2021/05/31/scm-integration/

The documentation is still sparse, but some integration can still be targeted to see how far it can come. Depending on the package structure and the possibilities, even if it turns out that the build would make sense be triggered for pushes only - that would be already good.