Update truffleruby-dev builds on Linux to Ubuntu 20.04

[nirvdrum]

Description

Currently, truffleruby-dev builds on Linux are based on Ubuntu 18.04. It's the oldest Ubuntu LTS release so it should present the fewest build issues. However, Ubuntu 18.04 will be EOL in a couple of months so we should switch to the truffleruby-dev builds based on Ubuntu 20.04. The TruffleRuby team will cease making snapshots based on Ubuntu 18.04 soon and the next GA release of TruffleRuby will not have Ubuntu 18.04 support.

This purely a change to the recipe file and the change should be transparent to users.