Arch mirror list should be ranked

When the primary mirror is too slow, pacman commands just fail, rather than falling back to the next available mirror. The preferred approach (by the community) is ranking the mirrors. We'll need the ranking code (available in the Arch repos), but for that we need pacman to work, so this approach is a catch-22 here. Perhaps we can think of a better solution here, that runs in the bare archlinux:latest container.