kdePackages.discover: don't build packagekit backend

Fixes #298092

pkgs/kde/plasma/discover/default.nix

···
 
   extraNativeBuildInputs = [pkg-config];
   extraBuildInputs = [qtwebview discount flatpak fwupd];
+
+  # The PackageKit backend doesn't work for us and causes Discover
+  # to freak out when loading. Disable it to not confuse users.
+  excludeDependencies = ["packagekit-qt"];
 }