Fix openssl build issues

Created by: exarkun

This bumps nixpkgs to a version where openssl 1.0.2 has the correct package hash (this was an upstream nixpkgs bug).

Of course this introduces its own new set of problems. The hash is only fixed in a revision where 1.1.1 has moved on to 1.1.1d which is incompatible with python-cryptography 2.7. So, also bump ZKAPAuthorizer revision to a version where python-cryptography is no longer being overridden. This lets the upstream nixpkgs python-cryptography package get used, which has also since been upgraded to 2.7 and had its tests patched to address the minor incompatibility.

Further, nixpkgs Python toolchain has stopped setting PYTHONHOME at all so stop cleaning it up.

And over the course of all of these changes, I found nixpkgs.nix to be more of a hindrance than a help so it is removed here and replaced by instructions to set NIX_PATH appropriately.