nix-env has its uses, but at least while you're learning it's better to put anything you need into systemPackages. Mainly because /etc/nixos should normally be

nix-env has its uses, but at least while you're learning it's better to put anything you need into systemPackages. Mainly because /etc/nixos should normally be a git repository. There isn't a good story for doing declarative package management with user packages (nix-env isn't it), though there are a hundred hacks.