feat(wpcarro/emacs): Support marcus.el

Sometimes I need device-specific Elisp. This is similar to what I'm doing in
google-briefcase (my gLinux-specific configuration).

Change-Id: Idc4031e74d31a91fb8d943acfaeb90b8742fbfc4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5084
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/nixos/marcus/default.nix

@@ -3,7 +3,10 @@
 
 let
   inherit (depot.users) wpcarro;
-  wpcarrosEmacs = wpcarro.emacs.nixos;
+
+  wpcarrosEmacs = wpcarro.emacs.nixos {
+    load = [ ./marcus.el ];
+  };
 
   quasselClient = pkgs.quassel.override {
     client = true;