Pin a particular nixpkgs

This gives us deterministic, portable (across time) builds w/ nix.

nixpkgs.nix

+{ pkgs ? import <nixpkgs> { } }:
+let
+  nixpkgs = fetchTarball
+  { url = "https://github.com/NixOS/nixpkgs-channels/archive/4557b9f1f50aa813ae673fe6fcd30ca872968947.tar.gz";
+    sha256 = "0cam48cn042axcik9vqxsqjc2hwyb2grjbjxacsn4w0y1zk6k6l2";
+  };
+  args =
+  { overlays = [ (import ./overlays.nix) ];
+  };
+in
+pkgs.callPackage nixpkgs args