Merge pull request #196224 from NickCao/qtwebengine

qt6.qtwebengine: correct how NIX_BUILD_CORES is propagated to the gn …

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

···
 
   requiredSystemFeatures = [ "big-parallel" ];
 
+  preConfigure = ''
+    export NINJAFLAGS="-j$NIX_BUILD_CORES"
+  '';
+
   postInstall = ''
     # This is required at runtime
     mkdir $out/libexec