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/tools/monzo_ynab/README.md

@@ -24,7 +24,7 @@ Or you can install using `nix-env` if you'd like to create the `monzo_ynab`
 symlink.
 
 ```shell
-> nix-env -f ~/briefcase/monzo_ynab -i
+> nix-env -iA users.wpcarro.monzo_ynab
 ```
 
 ## Deployment