install R on osx gitlab runner

[ben-schwen]

Current GL CI runner indicates that R might not be available

bash: line 152: Rscript: command not found

[jangorecki]

Was this change tested?
When working with CI I found it best to push to branch directly on gl so pipeline can run and test it. Submitting PR in gh and waiting for gl to pull recent master is not a nice way to go. Please try pushing directly to GL and see if it works. If it does, then we merge to master. So we don't use master for trying out.

[ben-schwen]

Thanks for the info! And the pipeline is also working like a charm for this :)

And without building manual its even green on CI!

[jangorecki]

So now that we have mac in CI we could as well add Mac binaries for r-devel, r-release and r-oldrel.