fix(wpcarro/nix): Remove <briefcase> references

Angle-bracketed references are a Nix anti-pattern, and thankfully this
repository enforces this as a standard.

TL;DR:
- Drop angle-bracketed references
- Change `briefcase` -> `users.wpcarro`
- Fix any resulting regressions
- Fix //users/wpcarro/tools/simple_vim
- Mark //users/wpcarro/boilerplate/typescript and related projects
  as broken
- drop .skip-subtree file, enabling depot CI

Change-Id: I7153cbabafa617bfd6b199370cbec65cb75441f6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4325
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: wpcarro <wpcarro@gmail.com>

users/wpcarro/tools/run/shell.nix

@@ -1,7 +1,6 @@
-let
-  briefcase = import <briefcase> {};
-  pkgs = briefcase.third_party.pkgs;
-in pkgs.mkShell {
+{ pkgs, ... }:
+
+pkgs.mkShell {
   buildInputs = with pkgs; [
     go
     goimports