ci: Use BuildJet Ubuntu 20.04 runners for better glibc compatibility (#18442)

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).