gistfile1.txt

$ nix-env -i nix-prefetch-git
$ nix-prefetch-git https://github.com/NixOS/nixpkgs.git > nixpkgs.json

gistfile2.txt

let
  bootstrap = import <nixpkgs> {};

  nixpkgs = builtins.fromJSON (builtins.readFile ./nixpkgs.json);

  nixpkgs_src = bootstrap.fetchFromGitHub {
    owner = "NixOS";
    repo  = "nixpkgs";
    inherit (nixpkgs) rev sha256;
  };

  nixpkgs_unstable = import nixpkgs_src {};

in {
  services.hydra = {
    package = nixpkgs_unstable.hydra;
    # (other config goes here)
  };
}