Add concurrency control to GitHub workflow runs

[davidanthoff]

If this works the way I think it works (and I've tested it a bit on another repo) then this would mean that if one pushes a new commit to a PR or branch that triggers a workflow run, any previous still running instance of the same workflow for that same PR or branch would be cancelled.

Should overall speed things up because one should run less often into resource constraints.

[codecov]

Codecov Report

Merging #712 (b397489) into master (e7d8b2d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #712   +/-   ##
=======================================
  Coverage   93.93%   93.93%           
=======================================
  Files           2        2           
  Lines         495      495           
=======================================
  Hits          465      465           
  Misses         30       30           

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[KristofferC]

Sounds good. Let's try it.