Stop using a nixpkgs fork.

We have a nixpkgs fork, since nixos-20.09 was the last version that enough working python2 packages to install our services. However, we can use mach-nix to build python2 derivations that work with at least nixos-21.05. Given that, continuing to maintain our nixpkgs fork represents an unnecessary maintenance burden:

• It is more work to update our packages, since we'd first need to bump them in our nixpkgs fork, and then in this repo.
• Given that the base version (nixos-20.09) is unsupported, we are responsible for any required security updates.