set default path separator to '/' in UCRT64 of MSYS

[leuven65]

Use path separator "/" also in UCRT64 of MSYS.

The environments of MSYS: [[https://www.msys2.org/docs/environments/]]

[tavianator]

The page you linked lists a few other environments. Rather than checking for each of them individually, should fd just check that $MSYSTEM is non-empty? I assume / should be used for all MSYS environments.

[CarterLi]

I also use both CLANG64 and CLANGARM64

[leuven65]

I have adapt the code to check if $MSYSTEM is non-empty.