Copy artifacts for schema tests after deployment (#2895)

After each deployment in sandbox or production, move the artifacts from
the corresponding release to a well-known location so that they can be
mapped to Kokoro in presubmit tests. The Kokoro-mapping does not need
public access to the GCS bucket.

The artifacts include the  postgresql schema jar, the nomulus release
jar, and the uber jar of the nomulus schema integration test classes.

Every jar name consists of a fixed prefix and the environment. Each jar
of a new deployment overrides the previous copy.

release/cloudbuild-schema-deploy.yaml

@@ -75,7 +75,7 @@ steps:
 # Save the deployed tag for the current environment on GCS to a well-known.
 # location. Do not use the 'artifacts' section for this since it will
 # upload an extra metadata file every time and pollute the folder.
-# TODO(weiminyu): modify this step so that TAG_NAME may be 'live'.
+# Also copy the schema jar here. It will be mapped into Kokoro for presubmit tests.
 - name: 'gcr.io/$PROJECT_ID/builder:latest'
   entrypoint: /bin/bash
   args:
@@ -84,6 +84,9 @@ steps:
     set -e
     echo ${TAG_NAME} | \
       gcloud storage cp - gs://$PROJECT_ID-deployed-tags/sql.${_ENV}.tag\
+    # Copy the schema jar here
+    gcloud storage cp gs://$PROJECT_ID-deploy/${TAG_NAME}/schema.jar \
+      gs://$PROJECT_ID-deployed-tags/schema-test-artifacts/schema.${_ENV}.jar
 timeout: 3600s
 options:
   machineType: 'E2_HIGHCPU_32'