Add ubuntu to minimal github ci workflow

[gh-tobiasdiez]

With the introduction of the WSL workflow, the question arose whether the build errors there are specific to WSL or are due to using Ubuntu 20.04 (e.g. #30629). To answer such questions, this ticket adds a pure-ubuntu workflow as comparison.

In the mid-term, a macos build should be added as well to this workflow. The idea would be then to check this workflow for each ticket before merging (this would probably require additional caching).

As usual, the code is provided via the following github PR for now: ​https://github.com/sagemath/sage/pull/99. After an initial review, I'll add it as a trac branch.

[mkoeppe]

Sage development has entered the release candidate phase for 9.3. Setting a new milestone for this ticket based on a cursory review of ticket status, priority, and last modification date.