add system-manager to nixpkgs

[thkoch2001]

Please (somebody) add system-manager to nixpkgs.

This would be a nice-to-have. Right now one needs to install nix and git on the target system to bootstrap system-manager.

This is mostly motivated by a healthy scepticism towards flakes.

[r-vdp]

We have a usable default.nix file as well, so you don't have to use flakes, even without the package being added to nixpkgs.