Build find-owners plugin on its own 3.1 branch

Change-Id: I5a46b32d1e47302159110a2be5177604dc2a10a0
diff --git a/jenkins/gerrit-bazel-plugin-find-owners.yaml b/jenkins/gerrit-bazel-plugin-find-owners.yaml
index 2f975c2..ff661bc 100644
--- a/jenkins/gerrit-bazel-plugin-find-owners.yaml
+++ b/jenkins/gerrit-bazel-plugin-find-owners.yaml
@@ -4,4 +4,5 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1
             - stable-2.16