Using a specific nixpkgs version to install a package

mypackage.nix

{ pkgs ? (import <nixpkgs> {}) }:

rec {
  # import nixpkgs from a specific commit hash
  oldNixpkgs = pkgs.fetchFromGitHub {
    owner = "nixos";
    repo = "nixpkgs";

    # here goes the commit hash
    rev = "c2268175651c4aa1da23c1b84ecebc0a0df56633";
    sha256 = "0asjhmzwgipbmv3l58l4g4miw6mhywpzzviirys1r33ykbacvjkf";
  };

  # import the "old" packages
  oldPkgs = import "${oldNixpkgs}" {};

  # use the oldPkgs and use the "ripgrep" package
  myRipgrep = oldPkgs.ripgrep;
}