My tip: don't use nix-env to install packages like it's Aptitude or Pacman, use `nix run` if you need a package temporarily, home-manager if you need a user-level package permanently, or just straight up change the configuration.nix to include the package as a system-wide one.

If you use nix-env you're just polluting your environment in an imperative way and that is pretty much going against Nix' declarative nature.