The GitHub Matrix-integration widget does these notifications,
more efficiently than running curl by hand.

This applies to the Debian nightly builds; having to go through
external actions doubles the effort to debug these kinds of
scripts, so just slap the actual commands into the workflow.
This also makes it easier to migrate to a different hosting.

- prep for the nightlies (only openSUSE affected)
- notifications (slightly more secure)
- fix unlabeling issues on close

- don't need multiple prepare steps
- try to use git output for SHA and log message

- expression context is not enough

- unclear why this wasn't evaluated in expression context before

- this should prevent forks from duplicate-reporting builds