Build readonly plugin on stable-3.4 branch

Change-Id: I3322d64c18121a0929def6e801c8ac3ddb791251

jenkins/gerrit-bazel-plugin-readonly.yaml

diff --git a/jenkins/gerrit-bazel-plugin-readonly.yaml b/jenkins/gerrit-bazel-plugin-readonly.yaml
index d60cb9a..958d1b0 100644
--- a/jenkins/gerrit-bazel-plugin-readonly.yaml
+++ b/jenkins/gerrit-bazel-plugin-readonly.yaml
@@ -6,6 +6,7 @@
             - master
           gerrit-branch:
             - master
+            - stable-3.4
             - stable-3.3
             - stable-3.2
       - 'plugin-{name}-bazel-{branch}':