CI: turn coverage into own action

[junghans]

#500 (comment)

[codecov]

Codecov Report

Merging #502 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #502   +/-   ##
======================================
  Coverage    52.7%   52.7%           
======================================
  Files         291     291           
  Lines       30330   30330           
======================================
  Hits        16012   16012           
  Misses      14318   14318           

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[junghans]

Hmm, the problem with two workflows is that the coverage workflow will still keep running, if the CI workflow already failed.

[junghans]

But writing them into one yaml as two jobs won't automatically cancel them either. We could make one job depend on the other using depend: just that would artificially serialize the execution.

So let's leave it like that for now until we find a better solution.