Increase build parallelism on macOS

[kkarbowiak]

According to this information, all the macOS runners have at least 3 cores, so it makes sense to increase the build parallelism to this number.

[codecov]

Codecov Report

Merging #2759 (ca1e2be) into devel (caffe79) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            devel    #2759   +/-   ##
=======================================
  Coverage   91.36%   91.36%           
=======================================
  Files         190      190           
  Lines        7855     7855           
=======================================
  Hits         7176     7176           
  Misses        679      679           

[horenmar]

Thanks, but since the time I wrote these I learned a better way to do this (see review).

[janisozaur]

While make is not as slow as some other build systems (cough msbuild cough), ninja is still faster and autodetects core count.

[horenmar]

I usually don't see ninja being faster in from-zero full builds (it is awesome for incremental dev builds though), and last time I tried it wasn't installed on the VMs by default so the build had to spend time installing it every time.

[janisozaur]

It seems the GitHub team is indeed opposed to installing ninja by default: actions/runner-images#8348