Actions running on `ubuntu-latest` cannot use sagemath

[oscarlevin]

Our tests run using the ubuntu-latest runner image. Unfortunately, this does not know about sagemath (it was installing it using apt.

It's probably time to switch to the docker-image (although I don't know how to do this). And probably time to install sage from source in the docker image (hopefully that takes less time than the 30 minute build time it takes to build sage from source).