[CI] GitHub Actions is deprecating `macos-10.5`

[straight-shoota]

We're currently using the macos-10.5 image in GHA which is deprecated and expected to be removed by end of August (https://github.com/actions/virtual-environments/issues/5583).

I have started the infrastructure update at https://github.com/crystal-lang/crystal/compare/master...straight-shoota:crystal:feature/ci-upgrade-macos?expand=1 Besides updating macos, some GHA actions needed an upgrade as well as nixpkgs in order to be compatible with macos-12.

But now there are some spec failures, which is really unexpected from an OS update. https://github.com/straight-shoota/crystal/runs/7558548524?check_suite_focus=true

I don't know what's going on there, but it looks like the executable path has changed somehow...?

Debugging on GHA is hard and I don't have access to MacOS hardware. Can someone using MacOS try to recreate the failures and figure out how to fix them?