conf-mingw-w64-gcc-i686 and conf-mingw-w64-gcc-x86_64 are not coinstallable on msys2

[kit-ty-kate]

cc @dra27

#25861 added these packages which each depend on msys2-mingw32 and msys2-mingw64. However these two packages are in a conflict-class together so they aren't coinstallable.

opam-core depends on both on Windows (although it's not exactly necessary but the configure script will fail otherwise) and the fact that both can't be installed at the same time means that opam-core can't be installed on msys2

[kit-ty-kate]

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