CI: Use Alpine instead of Ubuntu
Ubuntu 20.10 is now totally unsupported, and the Alpine tests should ensure better version coverage, and allow us to use specific versions of Dulwich instead.
This instance will be updated to Heptapod 18.6.0rc2 on 2025-12-11 between 17:00 and 18:00 UTC+1
Ubuntu 20.10 is now totally unsupported, and the Alpine tests should ensure better version coverage, and allow us to use specific versions of Dulwich instead.