Merging !139 (merged) allowed us to switch to the latest version of igraph, but in the process we broke CI, because we now need libigraph-dev-0.10.4 to be provisioned as a system dependency.

However, due to the fact that our CI doesn't use nix, our next best option would be to use apt-get install, but apparently there are no readily-available Ubuntu/Debian packages to install, so we are left with building this from source directly from CI.

However, building locally with --nix and in a "standard" way via CI is not good for reproducibility and consistency (see also #179 (closed) for an effort in a similar direction): ideally we should have only a single (reliable) way to build the project across all the environments.

The natural choice seems to be trying to improve CI by switching it to build via nix (inside the Docker container like we do now).

This relies on #188 (closed) being completed first.

Something to figure out is how to cache the nix store outside the container, to avoid rebuilding the world all the time. Perhaps we could also explore caching services like Cachix which gives us 5GB free for open source projects, but I have no clue if that's enough for us.