Improve downloading

[cstrahan]

Depending on the nix-prefetch-* scripts is a little fragile, since the flags change on occassion, as does the output format.

With the recent landing of https://github.com/NixOS/nix/commit/d367b8e7875161e655deaa96bf8a5dd0bcf8229e, the next release of Nix should allow us to do something like this instead:

nix-build --hash -E 'with import <nixpkgs> {}; fetchgit { url = "https://github.com/NixOS/nix.git"; sha256 = "ffffffffffffffffffffffffffffffffffffffffffffffffffff"; }'