refactor(wpcarro): Remove briefcase references

TL;DR:
- prefer WPCARRO env-var to BRIEFCASE
- remove repository URLs from Emacs libraries
- prefer tvl-depot-path where possible
- reduce the scope of constants.el
- prune (some not all) stale CI configuration

Change-Id: I21e9130402502ec6fa2fc4b46753c890069be62d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4545
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

users/wpcarro/emacs/README.md

@@ -5,9 +5,9 @@ as central to my workflow as `git` and `nix`.
 
 ## Installing
 
-If you already have `briefcase` on your local file system, run the following
-from the top-level `briefcase` directory:
+If you already have `depot` on your local file system, run the following from
+the top-level `depot` directory:
 
 ```shell
-$ nix-build -f . -iA emacs.nixos
+$ nix-env -iA users.wpcarro.emacs.nixos
 ```