ci: fix Python dependency on Ubuntu 24.04
Newer versions of Ubuntu have dropped Python 2 on newer versions of Ubuntu. By default though, our CI setups will try to use that Python version on all Ubuntu-based jobs except for the "linux-gcc" one, which consequently breaks those jobs.
Adapt our CI setup to only use Python 2 on Ubuntu 20.04 LTS, whereas we use Python 3 on all other Ubuntu jobs. Eventually, we should think about dropping support for Python 2 completely.
Closes CI fails due to missing Python 2 package starti... (#307 - closed).
Edited by Patrick Steinhardt