MinGW has started failing in main - https://github.com/python-pillow/Pillow/actions/runs/13051364081

The failure occurs with mingw-w64-x86_64-libimagequant-4.3.3-2, but did not occur with mingw-w64-x86_64-libimagequant-4.3.3-1.

MinGW does not allow installation of older versions of packages, so this PR stops installing libimagequant on MinGW for the moment.

As far as a possible long-term solution, I've created msys2/MINGW-packages#23245 to update the MinGW package to libimagequant 4.3.4, to see if that does anything.