nixos_config/system/nix_lix.nix

{
  nix.settings = {
    extra-substituters = [
    "https://cache.lix.systems"
    ];

    trusted-public-keys = [
      "cache.lix.systems:aBnZUw8zA7H35Cz2RyKFVs3H4PlGTLawyY5KRbvJR8o="
    ];
  };
}