how to get nix packages in path

{ config, pkgs, ... }:

{
  imports = [ <nixpkgs/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix> ];

  environment.systemPackages = with pkgs; [
    tmux
  ]
}

{ config, pkgs, ... }:

{
  imports = [ <nixpkgs/nixos/modules/installer/cd-dvd/installation-cd-minimal.nix> ];

  environment.systemPackages = with pkgs; [
    tmux
  ]
}

[nixos@nixos:~]$ sudo nixos-rebuild switch
building Nix...
building the system configuration...
[nixos@nixos:~]$ tmux
-bash: tmux: command not found

[nixos@nixos:~]$ sudo nixos-rebuild switch
building Nix...
building the system configuration...
[nixos@nixos:~]$ tmux
-bash: tmux: command not found