github/workflows: disable MSYS 32-bit builds

While mpv can be build for 32-bit platforms, MSYS started dropping some
of the 32-bit packages [1]. We cannot be asked to monitor and fix CI build
each time a dependency package randomly disappears [2].

Disable MSYS 32-bit builds if the support for this platforms is phased
out upstream. Sadly this will reduce our build coverage.

[1] https://www.msys2.org/news/#2023-12-13-starting-to-drop-some-32-bit-packages
[2] https://github.com/msys2/MINGW-packages/commit/fff2fa3711e3cf1c9b65b12784ab3d9428a779f1

See also: https://github.com/msys2/MINGW-packages/issues/19747