ci : add building in MSYS2 environments (Windows)

[przemoc]

Implements #1780.

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It will be failing until #1778 will get merged and this PR rebased then.

[przemoc]

Proof that it is passing on top of the other PR (+ disabled other jobs to speed up testing):
https://github.com/przemoc/whisper.cpp/commits/2de03c28f9b19493e18d021724610f97b38d2c22
https://github.com/przemoc/whisper.cpp/actions/runs/8429013889

[ulatekh]

I made a MinGW build in the process of looking at this project...CUDA isn't supported under MinGW, so there seems to be little point.

[przemoc]

Rebased and resolved very minor whitespace merge conflict.

@ulatekh, I think whisper.cpp started without any CUDA support at the beginning, so just because CUDA is not supported under MinGW-w64 doesn't mean it's not worth to have checks to ensure building in MSYS2 environments remains functional.

[przemoc]

After merging #1778, rebased one final time and, as expected, all is green now.