Remove default values for Nix expression parameters

I'm not sure if this commit breaks everything in my monorepo. I think it
will.

Why am I doing this? Perhaps it's a bad idea. I don't fully understand how
readTree works. My ignorance is costing me hours of time spent debugging. In an
effort to better understand readTree, I'm removing the default values for my Nix
expression parameters, which I believe have preventing errors from surfacing.

scratch/deepmind/part_two/shell.nix

@@ -1,4 +1,4 @@
-{ pkgs ? import <nixpkgs> {}, ... }:
+{ pkgs, ... }:
 
 pkgs.mkShell {
   buildInputs = with pkgs; [