Build multi-site plugin on stable-3.4 branch

Change-Id: I32ebdf4ea226b11a4dd88e77df93c091eb524537

jenkins/gerrit-bazel-plugin-multi-site.yaml

diff --git a/jenkins/gerrit-bazel-plugin-multi-site.yaml b/jenkins/gerrit-bazel-plugin-multi-site.yaml
index e0a5a9b..ff3fdb3 100644
--- a/jenkins/gerrit-bazel-plugin-multi-site.yaml
+++ b/jenkins/gerrit-bazel-plugin-multi-site.yaml
@@ -8,4 +8,9 @@
             - stable-3.2
             - stable-3.1
             - stable-2.16
+      - 'plugin-{name}-bazel-{branch}-{gerrit-branch}':
+          branch:
+            - master
+          gerrit-branch:
+            - stable-3.4
       - 'plugin-{name}-verifier'