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

tools/url-blocker/default.nix

@@ -1,8 +1,6 @@
-{ ... }:
+{ pkgs, ... }:
 
 let
-  pkgs = import <unstable> {};
-
   ghc = pkgs.haskellPackages.ghcWithPackages (hpkgs: [
     hpkgs.time
     hpkgs.aeson