xds-k8s buildscripts: Add missing quotation marks. (#27389)

These were missed when  creating the Python virtual env.
diff --git a/tools/internal_ci/linux/grpc_xds_k8s_install_test_driver.sh b/tools/internal_ci/linux/grpc_xds_k8s_install_test_driver.sh
index eea2066..ba3beaa 100644
--- a/tools/internal_ci/linux/grpc_xds_k8s_install_test_driver.sh
+++ b/tools/internal_ci/linux/grpc_xds_k8s_install_test_driver.sh
@@ -153,7 +153,7 @@
       echo "Found python virtual environment directory: ${venv_dir}"
     else
       echo "Creating python virtual environment: ${venv_dir}"
-      "python${PYTHON_VERSION} -m venv ${venv_dir}"
+      "python${PYTHON_VERSION}" -m venv "${venv_dir}"
     fi
     # Intentional: No need to check python venv activate script.
     # shellcheck source=/dev/null