Make nix module use nix rfc42 style settings

[Sohalt]

This changes the nix module to use RFC42 settings.

It's a backwards incompatible change, so not sure how you feel about it. It could theoretically also be done in a compatible way.

[eikek]

Hi @Sohalt thanks for this suggestion. I'm a bit torn tbh, because this then looses catching typos in the config. But it also removes a very tedious part of the work. I was already thinking of writing a conversion from the sharry default config to NixOS options, but this is not trivial :-) So I think less maintenance wins here 👍🏼

Can you possibly also change the configuration-test.nix file and update it accordingly? I think you can build it via nix build .#nixosConfigurations.test-vm.config.system.build.toplevel 🙏🏼 (It might not need an update, I didn't take a deeper look here).