qt6.qttools: drop incorrect references to CMAKE_INSTALL_INCLUDEDIR

pkgs/development/libraries/qt-6/modules/qttools.nix

@@ -23,6 +23,13 @@ qtModule {
   env.NIX_CFLAGS_COMPILE = toString [
     "-DNIX_OUTPUT_OUT=\"${placeholder "out"}\""
   ];
+  postPatch = ''
+    substituteInPlace \
+      src/qdoc/catch/CMakeLists.txt \
+      src/qdoc/catch_generators/CMakeLists.txt \
+      src/qdoc/catch_conversions/CMakeLists.txt \
+      --replace ''\'''${CMAKE_INSTALL_INCLUDEDIR}' "$out/include"
+  '';
   postInstall = ''
     mkdir -p "$dev"
     ln -s "$out/bin" "$dev/bin"