ci: Use pacboy to simplify installation of msys2 packages

ci: Use pacboy to simplify installation of msys2 packages

See [1] and [2] for reference on using pacboy to simplify installing
mingw packages in msys2. The relevant bit to know is that by
specifying plain package names with a ":p" suffix, in the "pacboy"
section in msys2/setup-msys2, those packages are installed within
the current msys2 environment.

This avoids needing to specify the long package prefix like
"mingw-w64-x86_64". For other environments like "ucrt64", "clang64"
and "clangarm64", the prefix is even longer and less obvious.

[1] https://github.com/msys2/setup-msys2?tab=readme-ov-file#pacboy
[2] https://www.msys2.org/docs/package-naming/#avoiding-writing-long-package-names