fishPlugins.hydro: add update script

pkgs/shells/fish/plugins/hydro.nix

@@ -2,6 +2,7 @@
   lib,
   buildFishPlugin,
   fetchFromGitHub,
+  unstableGitUpdater,
 }:
 
 buildFishPlugin {
@@ -15,6 +16,8 @@ buildFishPlugin {
     hash = "sha256-8ixve1ws80q5jNdKoooL25Lk7qopVitCMVTucW490fU=";
   };
 
+  passthru.updateScript = unstableGitUpdater { };
+
   meta = {
     description = "Ultra-pure, lag-free prompt with async Git status";
     homepage = "https://github.com/jorgebucaran/hydro";