message: pip.sh: unify the way virtualenv is invoked
  "python -m" also seems to be a more robust way of calling virtualenv
  than relying on the virtualenv command on path.
diff: @@ -296,19 +296,12 @@ create_activate_virtualenv_and_install_tensorflow() {
  die "FAILED to create virtualenv directory: ${VIRTUALENV_DIR}"
  fi
  - if [[ ${PYTHON_BIN_PATH} == *"python3.6"* ]]; then
  - "${PYTHON_BIN_PATH}" -m venv "${VIRTUALENV_FLAGS}" \
  + # Use the virtualenv from the default python version (i.e., python-virtualenv)
  + # to create the virtualenv directory for testing. Use the -p flag to specify
  + # the python version inside the to-be-created virtualenv directory.
  + python -m virtualenv -p "${PYTHON_BIN_PATH}" ${VIRTUALENV_FLAGS} \
  "${VIRTUALENV_DIR}" || \
  die "FAILED: Unable to create virtualenv"
  - else
  - # Verify that virtualenv exists
  - if [[ -z $(which virtualenv) ]]; then
  - die "FAILED: virtualenv not available on path"
  - fi
  - virtualenv ${VIRTUALENV_FLAGS} \
  - -p "${PYTHON_BIN_PATH}" "${VIRTUALENV_DIR}" || \
  - die "FAILED: Unable to create virtualenv"
  - fi
  source "${VIRTUALENV_DIR}/bin/activate" || \
  die "FAILED: Unable to activate virtualenv in ${VIRTUALENV_DIR}"
  