Ensure the right perl is used for TAP tests on msys
In particular, perl with $Config{osname} = msys should only be used if
the build target is msys (which is currently buildable but not usable).
For builds targeted at native Windows, perl from the ucrt64 toolchain is
suitable.
Discussion: https://postgr.es/m/
20220216210141.5glt5isg5qtwty4c@alap3.anarazel.de