Build spanner plugin on stable-3.13

The global-refdb module is needed since I4b798e367 to build the spanner
plugin.

Change-Id: Iea397f9fb91e97afcfbb37a9eb996b5f7db3c8a5

Author: barbasa

jenkins/gerrit-bazel-plugin-spanner-refdb.yaml

@@ -1,9 +1,9 @@
 - project:
     name: spanner-refdb
+    extra-modules: 'global-refdb'
     jobs:
-      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
+      - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
-          gerrit-branch:
-            - master
+            - stable-3.13
       - 'plugin-{name}-verifier'