Use BuildJet Ubuntu 20.04 runners. - Linux arm64 unchanged (glibc >= 2.35) - Linux x64 glibc requirement becomes to >= 2.31 (from glibc >= 2.35). Note: Ubuntu 20.04 repo cmake (3.16.3) is normally too old to build Zed, but `ubuntu-2004` [includes cmake 3.30.3](https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2004-Readme.md#tools).