Update netinstall_iso_finder for Ubuntu 20.04 focal -> legacy-installer
Ubuntu stops using the debian installer, but it is still available as legacy installer for focal 20.04 in http://archive.ubuntu.com/ubuntu/dists/focal/main/installer-amd64/current/legacy-images/
More details here: https://discourse.ubuntu.com/t/server-installer-plans-for-20-04-lts/13631