(windows-x64-mingw.yml) Use MXE build container for windows-x64 core builds
This PR switches the windows-x64-mingw.yml
CI template over to the new MXE build container.
DO NOT MERGE until the MXE container itself (https://git.libretro.com/libretro-infrastructure/libretro-build-mxe-win64-cross) has finished building, and has been tested.