Update mirrorlist using reflector in the live system upon establishing a network connection

Instead of downloading the mirrorlist when creating the iso and having the user manually sort it in the live system, it would be more useful to fetch it using reflector in the live system after establishing a network connection.

For Ethernet it should work using the reflector.service from the wiki. Though it's probably safer to use --age instead of --latest, e.g.:

ExecStart=/usr/bin/reflector --protocol https --age 1 --sort rate --save /etc/pacman.d/mirrorlist

For wireless it would need something like:

Restart=on-failure
RestartSec=?