kdePackages.mkKdeDerivation: move Qt Designer plugins out of $dev

Or they end up picked up by the build hooks

pkgs/kde/lib/mk-kde-derivation.nix

···
     defaultArgs = {
       inherit version src;
 
-      outputs = ["out" "dev"];
+      outputs = ["out" "dev" "devtools"];
 
       nativeBuildInputs = [cmake qt6.wrapQtAppsHook moveDevHook] ++ extraNativeBuildInputs;
       buildInputs = [qt6.qtbase] ++ extraBuildInputs;

pkgs/kde/lib/move-dev-hook.sh

···
 # shellcheck disable=SC2154
 
 moveKF6DevTools() {
-    moveToOutput "${qtPluginPrefix}/designer" "${!outputDev}"
+    if [ -n "$devtools" ]; then
+        mkdir -p "$devtools"
+        moveToOutput "${qtPluginPrefix}/designer" "$devtools"
+    fi
 }
 
 postInstallHooks+=('moveKF6DevTools')