ci: set up kokoro system tests to run on separate instances (#98)

Closes #83

Author: larkee

.kokoro/build.sh

@@ -29,6 +29,9 @@ export GOOGLE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/service-account.json
 # Setup project id.
 export PROJECT_ID=$(cat "${KOKORO_GFILE_DIR}/project-id.json")
 
+# Set up creating a new instance for each system test run
+export GOOGLE_CLOUD_TESTS_CREATE_SPANNER_INSTANCE=true
+
 # Remove old nox
 python3.6 -m pip uninstall --yes --quiet nox-automation