ci : add building in MSYS2 environments (Windows)

[przemoc]

I'm not familiar with GitHub Actions (yet), but it will be good to add most commonly used MSYS2 environments to CI for build sanity checks:

• UCRT64 (default since 2022-10-29, previously it was MINGW64 relying on old msvcrt.dll),
• CLANG64 (it's good to not rely on gcc only)
• CLANG32 (only if we want to have coverage in 32-bit builds too) - optional

MSYS2 does not support older Windows for quite some time already, so there is no point in targeting msvcrt-based environments (MINGW64 and MINGW32).

Environment details can be found on page:
https://www.msys2.org/docs/environments/

GitHub Action to setup MSYS2:
https://github.com/msys2/setup-msys2

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Depending on configuration (like enabling OpenBLAS), right now bulding in MSYS2 environments is not always working out of the box, but it was being improved in 2023, and is continued being improved directly (#1772) or indirectly (#1778).

Initially simple configurations (like w/o BLAS) could be built and in future expanded to cover more variants.

[przemoc]

Heads-up: I got windows-msys2 job in GitHub Actions successfully building whisper.cpp via make on UCRT64 and CLANG64 in my own branch on top of #1778, but will have to expand it to test not only via make but also via cmake and with OpenBLAS too (hence dependence on the other PR).

When I will have something relatively ready to share, I will create a dedicated PR.

[przemoc]

Forgot to close this issue after PR got merged.