Support for Apple M1

[elijahr]

A couple of fixes here that got omni working with my setup:

• clang on Apple M1 would not work with -march=native. I have added some logic to account for this. More info: https://stackoverflow.com/questions/65966969/why-does-march-native-not-work-on-apple-m1.
• Respect the NIMBLE_DIR environment variable which overrides ~/.nimble. This is important when using omni with a nim compiler installed via asdf-nim.
• Nim was complaining about --deadCodeElim being deprecated. I checked, and this behavior is always on since nim 0.19. Omni requires nim >= 1.4, so I have removed --deadCodeElim.

I've also updated the Github Actions workflow:

• Tests will now run when a pull request is submitted.
• The "All Tests - MacOS" job now uses asdf-nim, which reduces the build time from ~6 mins to under 2 mins. Example run: https://github.com/elijahr/omni/actions/runs/1624914076

[vitreo12]

Hey @elijahr!

Thanks a lot for the patch, I will look into it as soon as I can :)