Do not install libimagequant on MinGW

[radarhere]

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.

[hugovk]

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.

Thanks, this has been merged and mingw-w64-libimagequant is waiting to be built at https://packages.msys2.org/queue to update from 4.3.3-2 to 4.3.4-1.

Let's wait a bit to see if it's working when done.

[hugovk]

https://packages.msys2.org/packages/mingw-w64-x86_64-libimagequant now shows 4.3.4-1 but we're still getting:

resolving dependencies...
looking for conflicting packages...
...
Packages (83) [...] mingw-w64-x86_64-libimagequant-4.3.3-2 [...]
...
mingw-w64-x86_64-libimagequant-4.3.3-2-any downloading...

I guess one of the packages is depending on the old one?

Anyway, let's merge this to get the CI back to green, and revisit later.

Thanks!

[radarhere]

4.3.4-1 now appears in GitHub Actions, but it doesn't resolve the problem.

I've created msys2/MINGW-packages#23316 to see if anyone over there can assist.

[radarhere]

msys2/MINGW-packages#23316 was fixed by msys2/MINGW-packages#23387, and I've now created #8778 to revert this.