Isolate builds per PKGBUILD

[lazka]

To reduce the blast radius of a malicious package, it would be nice if it could only modify its own content and not the content of any other package that happens to be built alongside it.

The problem is that this would create lots of jobs and it's hard to build packages that depend on each other one after the other.
GitHub Actions doesn't allow creating dynamic dependencies between generated jobs. The only thing we can do is add multiple stages and decide which jobs get run in which stages.

So we could:

• One job per build type per PKGBUILD (so, lots of jobs..)
• First stage contains all jobs that can be built right away
• Second stage contains all jobs that depend on something in stage 1

tricky..

[lazka]

To avoid creating too many jobs/runs we could dynamically create multiple runs during one build job/run.
A first step in that direction is to merge the schedule with the supervise job.

edit: merge is done now