[CI] Re-enable test_darwin on circle-ci

[straight-shoota]

This re-activates test_darwin job on circle-ci as a temporal workaround for #10110.

We were using this job before but it broke with LLVM 10 and got disabled in #9763. #10078 finally removed the entire test_all_platforms workflow on circleci because all PR-jobs were moved to GitHub Actions.

Now the job works again and the nix-based alternative on GitHub Actions is broken instead.

We can see that is works because the job did not succeed, since master is broken on macos (#10475) which wasn't reported by the GitHub Action job.

[straight-shoota]

Until we figure out how to reliably make nix-shell errors fail the build (#10110), we should also reactive test_darwin on circle-ci.
This is probably even more important than #10480.

[straight-shoota]

Oh well, this was actually already merged with #10479 but it's broken again because brew setup needs some extra steps now (see 088fa18).

[bcardiff]

Note that on_osx will not exec the command on github ci: on_osx, on_linux, and on_nix_shell are mutually exclusive.

I think you are not expecting this.

[straight-shoota]

No, I am aware of that. The fix is intended for circleci because that's breaking without the last commit (https://circleci.com/gh/straight-shoota/crystal/3069?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link).

Github Actions doesn't use brew currently. This migth affect #10480 though.

[bcardiff]

Oh. Ok. The mention of GitHub in the comment confused me.