Update workflow to be used in merge queue.

[programmiri]

Resolves: #2419

Note:
This is more complicated as expected unfortunately, as having a workflow that runs only in the queue is not straightforward.

Currently, what I read up and tested, this is how it should work:

-> workflow is triggered on PR event AND merge_group event
-> jobs only should run when it's really queued (not for e.g. opening a PR)
-> job still needs to be triggered by required checks AND required merge groups

This does not seem to work as expected, though, because while the last test run only merged after the required jobs where done, it seems like on the test-main branch another job failed later. This could have been due to timing though and needs more investigation.

Will keep this Draft open for now, but it's not merge ready yet.

[programmiri]

Closing this, as there's currently no time to work on it & the repo is not super active, so we can stay at our current pipeline for now.