arch-nspawn: Generalize mirrorlist sed arch and repo variable replacement

Created by: tylerjl

The host_mirrors value uses a sed pattern that is closely coupled with the mirror URL pattern in-use by most Arch mirrors. For some variants, like Arch Linux ARM, values such as $repo and $arch sit in other parts in the mirror path. This change makes the sed replacement more generic to accommodate for variations in mirrorlist URLs while remaining backward compatible.

I ran into this when trying to use some downstream tools that rely on arch-nspawn (aurutils) and these changes seem to resolve things for my aarch64 and armv7 hosts.