Build avatars-external against gerrit 3.3

Change-Id: I89e1cd66146b25f01c4c17440b2defe1d5662589
diff --git a/jenkins/gerrit-bazel-plugin-avatars-external.yaml b/jenkins/gerrit-bazel-plugin-avatars-external.yaml
index 54782e3..bee173b 100644
--- a/jenkins/gerrit-bazel-plugin-avatars-external.yaml
+++ b/jenkins/gerrit-bazel-plugin-avatars-external.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.3
             - stable-3.2
             - stable-3.1
       - 'plugin-{name}-bazel-{branch}':