Reduce number of build threads on MAC

.github/workflows/mac.yml

@@ -21,8 +21,8 @@ jobs:
           mkdir build
           cd build
           cmake -GNinja -DDD4HEP_USE_GEANT4=ON -DBoost_NO_BOOST_CMAKE=ON -DDD4HEP_USE_LCIO=ON -DBUILD_TESTING=ON -DDD4HEP_USE_XERCESC=ON -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_STANDARD=17 ..
-          ninja
-          ninja install
+          ninja -j 2
+          ninja install -j2
           . ../bin/thisdd4hep.sh
           ctest --output-on-failure -j4
           cd ../examples/