How to pin a nix version in a default.nix file. (Override the pinned version via: nix-shell --arg nixpkgs '<nixpkgs>')

default.nix

{nixpkgs ? null}:
let
  sysPkg = import <nixpkgs> { };
  pinnedPkg = sysPkg.fetchFromGitHub {
    owner = "NixOS";
    repo = "nixpkgs";
    rev = "cf62a27a326a9cdbb01d627b1faaea3a0d5e886c";
    sha256 = "1q74ciqq5r17vidw2025x8idsncz9k7w60warrij4q7piaffgvd7";
  };
  pkgs = if nixpkgs==null then
           import pinnedPkg {}
         else
           import nixpkgs {};
in with pkgs; stdenv.mkDerivation rec {
  name = "ProjectA-1.0.0";
  src = ./.;
  buildInputs = [ pkgs.python ];
  installPhase= ''
    sed -i -e "s|python|`which python`" bin/serve
    mkdir -p $out/bin
    cp bin/* $out/bin
  '';
}