how to add unstable correctly to config.nix

gistfile1.txt

{
  allowUnfree = true;

  packageOverrides = pkgs_: {
      unstable = import <nixpkgs-unstable> { 
        # pass the nixpkgs config to the unstable alias
        # to ensure `allowUnfree = true;` is propagated:
        config = config.nixpkgs.config; 
     };
    let jdk = openjdk11; in {

    all = with pkgs; buildEnv {
      name = "all";

      paths = [
        htop