Mingw fluid~ fix
Second attempt to fix #690 (closed), which was a bit more work than I originally anticipated.
@jwilkes IMPORTANT: This requires mingw-w64-i686-SDL2 (which is now required by fluidsynth, but the fluidsynth package for some reason lacks a dependency on it) to be added to the Windows runner to work. So a manual modification of the Windows runner is needed: pacman -S mingw-w64-i686-SDL2