Use Zizmor for checking config GitHub Actions CI

[PhilippWendler]

There are many things that one can do wrongly when configuring GitHub Actions, and some default settings are not as good as they should be. The tool Zizmor checks and warns for common patterns.

I recommend JavaSMT to apply Zizmor, as BenchExec now does as well. You can see how BenchExec does it here: sosy-lab/benchexec@7654a2b