Set up CI system to replicate Homebrew installation on multiple toolchains

[abotkin-cpi]

As exhibited by the issues with Xcode 13 & #67, we need to update our CI setup so that we're directly replicating a Homebrew installation as part of the commit checks (and ensure all PRs auto run this & require the check to pass). We should additionally ensure it tries a command or two to ensure no runtime errors.