feat(fun/uggc): Install desktop handler in Nix derivation

This actually registers the desktop entry, so xdg-open etc. start
working:

tazjin@frog /depot> xdg-open 'uggcf://gjvggre.pbz/alnabgrpu/fgnghf/1272363652679524352'
Opening in existing browser session.

Change-Id: Ia754cb87470fce2f8d3340d1147f3f01b9914858
Reviewed-on: https://cl.tvl.fyi/c/depot/+/387
Reviewed-by: riking <rikingcoding@gmail.com>

users/tazjin/nixos/frog/default.nix

@@ -189,9 +189,10 @@ in depot.lib.fix(self: {
   environment.systemPackages =
     # programs from the depot
     (with depot; [
-      fun.idual.script
-      lieer
       frogEmacs
+      fun.idual.script
+      fun.uggc
+      lieer
       ops.kontemplate
       third_party.ffmpeg
       third_party.git