Build avatars-external plugin on stable-3.4 branch

Change-Id: I5e66832b5a0aa37da99663d78ab8d331de96bebc
diff --git a/jenkins/gerrit-bazel-plugin-avatars-external.yaml b/jenkins/gerrit-bazel-plugin-avatars-external.yaml
index bee173b..125a0df 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-3.4
             - stable-3.3
             - stable-3.2
             - stable-3.1