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 upgraded to Heptapod 18.0.1 on 2025-07-10 between 14:00 and 15:00 UTC+2
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.