feat: macOS build+test action

[SecondFlight]

This patch adds a build+test action for macOS.

Passing build here: https://github.com/SecondFlight/rid/pull/5

[thlorenz]

Awesome I very much like it!

Ideally we'd add this directly to the main build yaml along the lines of this?
I'm already using matrix builds and you can adapt the installation of things like make by defining vars for each OS as shown in the example I linked.

An alternative, but not as desirable would be splitting install tasks out and using runs-on to use the proper ones for each OS. This we should use if something cannot be solved via env vars.

Rough example:

ubuntu-deps:
    runs-on: ubuntu-latest
    steps:
     .....
macos-deps:
    runs-on: macos-latest
    steps:
     .....

Either approach would avoid duplicating other parts of the build config in several files and having to update them in multiple places if they change.

LMK if you need help with this and thanks!

[SecondFlight]

Closing in favor of #38.