Build aws-dynamodb-refdb plugin on stable-3.13

Change-Id: I013dc26f8de0da685bfec41b8f364d0f8ebc7a54

Author: barbasa

jenkins/gerrit-bazel-plugin-aws-dynamodb-refdb.yaml

@@ -12,5 +12,6 @@
       - 'plugin-{name}-gh-bazel-{branch}':
           branch:
             - master
+            - stable-3.13
       - 'plugin-{name}-verifier'
       - 'plugin-{name}-verifier-gh'