Remove msys from CI and install fypp via msys2 package

[minhqdao]

• Remove msystem: msys from CI.
• Install fypp via msys2 package instead of pip.

Address #853.

[zoziha]

@minhqdao, thank you for your sharing.

As far as I know, msystem:MSYS does not have the fypp package. In this PR, msystem:MSYS will actually use the tools of the msystem:MINGW64 environment, that is to say, msystem:MSYS has no actual effect for CI testing. Combined with the topic of #853, msystem:MSYS should be removed.

I have been a user of MSYS2 for few years. I believe that msystem:MSYS was originally only used to provide an initial Unix-like compilation environment and tools for other environments, and almost no one will use it as a development environment.

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It is sufficient for Stdlib to use msystem:MINGW64, msystem:UCRT64, and msystem:CLANG64. MSYS2 also plans to abandon the 32-bit environment, so we should remove the tests of msystem:MSYS and the 32-bit environment (maybe later); Since Flang and LFortran in the msystem:CLANG64 environment are not ready yet, CLANG64 does not need to be supported at present.

[minhqdao]

I see, it was probably not the best idea to merge the two setups then – I can revert that. In fact, the idea for this PR was only to fix the setup for msystem: mingw64. So we can still do that and leave msystem: msys as is for now because it is still working. If there's a consensus on removing msys entirely, I'm happy to do that in a separate PR.

[minhqdao]

Ok, I see it was msystem: msys which failed. Let's probably remove it then.

[zoziha]

@minhqdao, thanks, LGTM.

[minhqdao]

Will merge this so the CI stops failing. If there's ever the need to bring back msystem: msys, we can do #853 (comment).