Revert "pacman: force .tar.xz again"
Revert "pacman: force .tar.xz again"

This reverts commit e27632cfd490a625b81bf4b7f7535b0ae1e3ad7e.

The cause was the old outdated appveyor image. This will likely never change, so
we might as well force the issue now.

atom2013/MSYS2-packages,Alexpux/MSYS2-packages,mati865/MSYS2-packages,waterlan/MSYS2-packages,mati865/MSYS2-packages,JPeterMugaas/MSYS2-packages,waterlan/MSYS2-packages,JPeterMugaas/MSYS2-packages,Alexpux/MSYS2-packages,JPeterMugaas/MSYS2-packages,mati865/MSYS2-packages,mati865/MSYS2-packages,JPeterMugaas/MSYS2-packages,waterlan/MSYS2-packages,atom2013/MSYS2-packages,Alexpux/MSYS2-packages,Alexpux/MSYS2-packages,waterlan/MSYS2-packages,atom2013/MSYS2-packages,atom2013/MSYS2-packages