Nix is so nice that you can put almost your entire workflow into a check or package. Like your code-coverage report step(s) become a package that you build (I'm not brave enough to do this)

I run my own jenkins for personal stuff on top of nixos, all jobs run inside devenv shell, devenv handles whatever background services required (i.e. database), /nix/store is shared between workers + attic cache in local network.

Oh, and there is also nixosModule that is tested in the VM that also smoke tests the service.

First build might take some time, but all future jobs run fast. The same can be done on GHA, but on github-hosted runners you can't get shared /nix/store.