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

website/sandbox/contentful/.envrc

@@ -1,3 +1,4 @@
 source_up
+use_nix
 export CONTENTFUL_SPACE_ID="$(pass show programming/contentful/space-id)"
 export CONTENTFUL_ACCESS_TOKEN="$(pass show programming/contentful/access-token)"

website/sandbox/contentful/default.nix

@@ -1,6 +1,6 @@
-let
-  pkgs = import <nixpkgs> {};
-in pkgs.stdenv.mkDerivation {
+{ pkgs, ... }:
+
+pkgs.stdenv.mkDerivation {
   name = "ideal-website";
   src = builtins.path { path = ./.; name = "contentful"; };
   buildInputs = with pkgs; [

website/sandbox/contentful/shell.nix

@@ -1,5 +1,6 @@
 let
-  pkgs = import <nixpkgs> {};
+  briefcase = import <briefcase> {};
+  pkgs = briefcase.third_party.pkgs;
 in pkgs.mkShell {
   buildInputs = with pkgs; [
     nodejs

website/sandbox/covid-uk/shell.nix

@@ -1,5 +1,6 @@
 let
-  pkgs = import <nixpkgs> {};
+  briefcase = import <briefcase> {};
+  pkgs = briefcase.third_party.pkgs;
 in pkgs.mkShell {
   buildInputs = with pkgs; [
     yarn

website/sandbox/learnpianochords/.envrc

@@ -0,0 +1,2 @@
+source_up
+use_nix

website/sandbox/learnpianochords/dir-locals.nix

@@ -1,3 +0,0 @@
-let
-  briefcase = import <briefcase> {};
-in briefcase.utils.nixBufferFromShell ./shell.nix

website/sandbox/learnpianochords/shell.nix

@@ -2,9 +2,9 @@ let
   briefcase = import <briefcase> {};
   pkgs = briefcase.third_party.pkgs;
 in pkgs.mkShell {
-  buildInputs = with pkgs; [
-    elmPackages.elm
-    elmPackages.elm-format
-    elmPackages.elm-live
+  buildInputs = with pkgs.elmPackages; [
+    elm
+    elm-format
+    elm-live
   ];
 }