PR validation builds aren't affected, which is why this wasn't caught before scala/scala#7432 was merged

btw I did check that bionic is an upgrade, not a downgrade. bionic is ubuntu 18, xenial was ubuntu 16. so I think this isn't wasted work as we'd likely need to upgrade eventually

GitHub Actions has already been on 18 for a while and is presently moving to 20, as per actions/runner-images#1816