message: kokoro: Use gcc-7 for now.
diff: @@ -65,7 +65,7 @@ sudo update-alternatives --install /usr/bin/cpp cpp-bin /usr/bin/cpp-8 100
  sudo update-alternatives --install /usr/bin/cpp cpp-bin /usr/bin/cpp-7 50
  sudo update-alternatives --install /usr/bin/cpp cpp-bin /usr/bin/cpp-6 25
  # set alternatives
  -sudo update-alternatives --set g++ /usr/bin/g++-9
  -sudo update-alternatives --set gcc /usr/bin/gcc-9
  -sudo update-alternatives --set cpp-bin /usr/bin/cpp-9
  +sudo update-alternatives --set g++ /usr/bin/g++-7
  +sudo update-alternatives --set gcc /usr/bin/gcc-7
  +sudo update-alternatives --set cpp-bin /usr/bin/cpp-7
  echo "----------------------------------------"
  