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

.envrc

@@ -1,10 +1,8 @@
-export BRIEFCASE="$(realpath ~/briefcase)"
-export DEPOT="$(realpath ~/depot)"
-export NIXPKGS="$(realpath ~/nixpkgs)"
-# I'm ensuring that $NIX_PATH is unset to avoid the anti-pattern of depending on
-# <nixpkgs>, <briefcase>, <depot>, <unstable>.
-# For more information about setting this to "", see:
+export BRIEFCASE="$(realpath .)"
+# I'm ensuring that $NIX_PATH is mostly empty, so that I only depend on
+# <briefcase> for now.
+# For more information on the NIX_PATH anti-pattern, see here:
 # https://nix.dev/tutorials/towards-reproducibility-pinning-nixpkgs.html#pinning-nixpkgs
-export NIX_PATH="";
+export NIX_PATH="briefcase=$BRIEFCASE";
 export DESKTOP="zeno.lon.corp.google.com";
 export LAPTOP="seneca";