Drop support for dir-locals.nix, <nixpkgs>, etc.

In the spirit of Marie Kondo, I'm tidying up!

TL;DR:
- Prefer .envrc `use_nix` and delete all dir-locals.nix files
- Remove ~all references to <nixpkgs>, <unstable>, <depot> and prefer
  referencing each with briefcase.third_party.{pkgs,unstable,depot}
- Delete nixBufferFromShell function since I was only using that in
  dir-locals.nix files

playbooks/nix_gcr/cloud_run.nix

@@ -1,8 +1,4 @@
-{
-  pkgs ? import <nixpkgs> {},
-  depot ? import <depot> {},
-  ...
-}:
+{ pkgs, depot, ... }:
 
 pkgs.dockerTools.buildLayeredImage {
   name = "gemma";