utils/env: improve migw detection on windows

When running `python` within the various shells made available on 
Windows by MSys2, `sysconfig.get_platform()` returns:

* MSys2         : `msys-3.2.0-x86-64`
* MinGW x32     : `mingw_i686`
* MinGW x64     : `mingw_x86_64`
* MinGW UCRT64  : `mingw_x86_64_ucrt`
* MinGW Clang64 : `mingw_x86_64_clang`

In the case that `sysconfig.get_platform()` *does* return `mingw` (with
no suffix), this change should still work as expected.

Related-to: #3713