greetd: add update script

pkgs/by-name/gr/greetd/package.nix

···
   pam,
   scdoc,
   installShellFiles,
+  nix-update-script,
   # legacy passthrus
   gtkgreet,
   qtgreet,
···
     }
     // {
       greetd = warnPassthru "greetd" finalAttrs.finalPackage;
+      updateScript = nix-update-script { };
     };
 
   meta = {