nix-src = pkgs.fetchFromGitHub
   { owner = "NixOS";
     repo = "nix";
     rev = "2e953b567ebbcd3aaaf71bf592ab733bdcdd6797";
     sha256 = "009f8dha87vqmbfdn5xj68inv8bg73sr5000mk740fq4gmh681dq";
     fetchSubmodules = true;
   };

...

  package = (import "${nix-src}/release.nix" {}).build.x86_64-linux;

...

fatal: '/nix/store/8nnbnhj32z33gr7a4ix3z6fqw7xh1nwc-source' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
error: program 'git' failed with exit code 128
(use '--show-trace' to show detailed location information)
building Nix...
fatal: '/nix/store/8nnbnhj32z33gr7a4ix3z6fqw7xh1nwc-source' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
error: program 'git' failed with exit code 128
(use '--show-trace' to show detailed location information)
building the system configuration...
trace: warning: types.string is deprecated because it quietly concatenates strings
fatal: '/nix/store/8nnbnhj32z33gr7a4ix3z6fqw7xh1nwc-source' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
error: program 'git' failed with exit code 128
(use '--show-trace' to show detailed location information)