First attempt: use selfhosted macOS runner

[aaruni96]

Currently 4 runners with 5 workers per runner.

Probably needs a review by a Github actions expert

cc @fingolfin @benlorenz

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.16%. Comparing base (c2ea751) to head (bcee19b).
Report is 42 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3650      +/-   ##
==========================================
+ Coverage   82.78%   83.16%   +0.37%     
==========================================
  Files         577      579       +2     
  Lines       77755    78474     +719     
==========================================
+ Hits        64370    65263     +893     
+ Misses      13385    13211     -174     

see 74 files with indirect coverage changes

[fingolfin]

If I hear no concerns or objections by @benlorenz (or anyone else) I'll merge this tomorrow

(CC @fieker @thofma)