appveyor: don't try to build for mingw

pacman is giving me:
:: msys2-runtime and catgets are in conflict.
Remove catgets? [y/N] error: unresolvable package conflicts detected
error: failed to prepare transaction (conflicting dependencies)
2c697e50dbAndrew Kelley committed on 2/9/2018, 6:15:17 AMparent5911962
1 file changedLine totals unavailable