Make the speed up of compilation automatic on Linux

tools/run_tests/helper_scripts/build_python.sh

@@ -122,7 +122,8 @@ export LANG=en_US.UTF-8
 
 # Allow build_ext to build C/C++ files in parallel
 # by enabling a monkeypatch. It speeds up the build a lot.
-export GRPC_PYTHON_BUILD_EXT_COMPILER_JOBS=${GRPC_PYTHON_BUILD_EXT_COMPILER_JOBS:-4}
+DEFAULT_PARALLEL_JOBS=`nproc` || DEFAULT_PARALLEL_JOBS=4
+export GRPC_PYTHON_BUILD_EXT_COMPILER_JOBS=${GRPC_PYTHON_BUILD_EXT_COMPILER_JOBS:-$DEFAULT_PARALLEL_JOBS}
 
 # If ccache is available on Linux, use it.
 if [ "$(is_linux)" ]; then