chore: Implement code review suggestions

shell.nix

@@ -1,6 +1,15 @@
+# Use npins to pin nixpkgs dependencies
+# This makes it easier to cache build results
 {
-  pkgs ? import <nixpkgs> { },
+  sources ? import ./npins,
+  system ? builtins.currentSystem,
+  pkgs ? import sources.nixpkgs {
+    inherit system;
+    config = { };
+    overlays = [ ];
+  },
 }:
+# Create a nix shell with devenv (so `use devenv` can work in direnv/.envrc)
 pkgs.mkShell {
   buildInputs = with pkgs; [
     devenv