Pin nixpkgs for building the libchallenge_bypass_ristretto_ffi dependency separately

[exarkun]

As of #106 the nixpkgs from haskell.nix is used to build libchallenge_bypass_ristretto_ffi. However, that nixpkgs is updated on a schedule to suite Haskell development and the haskell.nix project. We may want to update nixpkgs for our non-Haskell dependencies on an independent schedule.

Make this possible by managing a nixpkgs source separately from the haskell.nix source.