That worked out nicely. Thanks a lot.
I have also found this thread which is basically what I am trying to do. But instead of preloading the built OS closure into a live-iso I am trying to host it on my own nix cache.
If I want to install that system I will probably have to call nixos-install with the --system option and give it the path of the built closure from the above nix-build command, right?
Or does it just compare the config hashes and work out the system path on its own?