Auto merge of #121182 - majaha:mingw_ci_new, r=Mark-Simulacrum

Improvements to building and CI for mingw/msys

I was getting error messages when trying to follow the build instructions the mingw build for Rust, and managed to track the issue down to an incomparability of Rust's bootstrap program with MSYS2's version of git. Essentially, the problem is that MSYS2's git works in emulated unix-y paths, but bootstrap expects a Windows path. I found a workaround for this by using relative paths instead of absolute paths.

Along with that fix, this PR also updates the build instructions for MinGW to be compatible with modern versions of MSYS2, and some changes to CI to make sure that MSYS2's version of git is tested. In particular, I'm suggesting using the [MSYS2 github action](https://github.com/marketplace/actions/setup-msys2) specially made for this purpose, which is much less hacky than the old approach and gives us more control of what packages are installed. I also cleaned up as many alternate versions of key tools as I could find from PATH, to avoid accidental usage, and cleaned up some abuses of the `CUSTOM_MINGW` environment variable.

This fixes rust-lang/rust#105696 and fixes rust-lang/rust#117567

Author: bors