chore: update system.sh with firestore creds.json path (#15599)

This is a blocker for the migration of firestore to this repo. Firestore
needs access to a project that has firestore native DBs, which
precise-truck-742 does not explicitly have.

Fixes failing tests in
#15590 🦕

Author: chalmerlowe

.kokoro/system.sh

@@ -21,6 +21,9 @@ set -eo pipefail
 # Disable buffering, so that the logs stream through.
 export PYTHONUNBUFFERED=1
 
+# Setup firestore account credentials
+export FIRESTORE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/firebase-credentials.json
+
 # Setup service account credentials.
 export GOOGLE_APPLICATION_CREDENTIALS=${KOKORO_GFILE_DIR}/service-account.json