CI: Add concurrency

[larsoner]

Closes #958

[codecov]

Codecov Report

Merging #959 (276ddd5) into main (6711c69) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #959   +/-   ##
=======================================
  Coverage   94.51%   94.51%           
=======================================
  Files          25       25           
  Lines        3573     3573           
=======================================
  Hits         3377     3377           
  Misses        196      196           

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[sappelhoff]

nice, thanks!

[hoechenberger]

Thanks @larsoner!