Fix macos kokoro builds (#4985)

kokoro/scripts/macos/build.sh

@@ -43,11 +43,9 @@ mkdir build && cd $SRC/build
 # Invoke the build.
 BUILD_SHA=${KOKORO_GITHUB_COMMIT:-$KOKORO_GITHUB_PULL_REQUEST_COMMIT}
 echo $(date): Starting build...
-# We need Python 3.  At the moment python3.7 is the newest Python on Kokoro.
 cmake \
   -GNinja \
   -DCMAKE_INSTALL_PREFIX=$KOKORO_ARTIFACTS_DIR/install \
-  -DPYTHON_EXECUTABLE:FILEPATH=/usr/local/bin/python3.7 \
   -DCMAKE_C_COMPILER=clang \
   -DCMAKE_CXX_COMPILER=clang++ \
   -DCMAKE_BUILD_TYPE=$BUILD_TYPE \