[RFC] Add a simpler pacboy alternative for installing mingw packages

With the goal to have a build matrix where you only vary MSYSTEM and the rest of the config is the same, one either needs to also specify the package prefix or use the pacboy option.

I was wondering if we could provide an additional action option like "msystem_install" which takes a list of packages and just prefixes them with "$MINGW_PACKAGE_PREFIX", merging them with the existing "install" option. In case of "MSYS" it would be the same as "install". It might be easier to understand and doesn't require an extra tool to know about.

    - uses: msys2/setup-msys2@v2
      with:
        msystem: UCRT64
        install: git make
        msystem_install: openssl curl

Another option might be to have some templating and do something like install: git @MINGW_PACKAGE_PREFIX@-openssl. But I'm worried that with env vars, github/matrix vars, another "variable" type specific to this action might add more confusion.

Start by reviewing how the action currently handles the install option and MSYSTEM, then trace how the MINGW_PACKAGE_PREFIX is applied or exposed. Compare the proposed msystem_install behavior with pacboy across the listed MSYSTEM values, and confirm the option's semantics and tests before implementation.