feat(wpcarro/marcus): Ensure EDITOR is set to Emacs

I should've done this awhile ago. I'm tired of having `nano` open in vterm
instances.

Change-Id: I784e4a7a3e8acba11105052b6c0fe1f6f9ac614b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4512
Reviewed-by: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/emacs/default.nix

@@ -1,5 +1,7 @@
 { pkgs, depot, ... }:
 
+# TODO(wpcarro): See if it's possible to expose emacsclient on PATH, so that I
+# don't need to depend on wpcarros-emacs and emacs in my NixOS configurations.
 let
   inherit (builtins) path;
   inherit (depot.third_party.nixpkgs) emacsPackagesGen emacs27;