CI: Use Docker runner for NixOS job
This uses the nixos/nix
Docker image on a standard Docker CI runner to
execute the Nix build job. We already tested and reviewed this in !134 (merged) -
it does not greatly increase the pipeline runtime. Another advantage is
that it validates the build without an active Nix cache.
It is exactly the code from !134 (merged) before applying my suggestion in !134 (comment 1651912318)
Closes: #158 (closed)
Edited by Felix Walter