Rollup merge of #136975 - jyn514:macos-x, r=Mark-Simulacrum

Look for `python3` first on MacOS, not `py`

`py` is not installed by default *and* trying to run it results in a popup asking if you want to install it. `python3` is installed by default.

This hopefully should not be too disruptive to people on Windows, since they should be going through `x.ps1` instead anyway. Just in case, I've added a check for Cygwin and Msys (i'm not sure how else you'd get a bash shell on windows).

I've tested this on macOS, WSL, "git bash" (which appears to be MSYS), and MSVC with `bash -c ./x` (which runs in WSL, but differently than interactive bash 🙃). I haven't tested Cygwin, which is unsupported anyway, but the code tries to make it work.

Author: jieyouxu