Build github plugin on stable-3.13

Change-Id: Idf23afeb5414ce951ad3468297b669961b2fb41d
diff --git a/jenkins/gerrit-bazel-plugin-github.yaml b/jenkins/gerrit-bazel-plugin-github.yaml
index fc22ad4..73e5926 100644
--- a/jenkins/gerrit-bazel-plugin-github.yaml
+++ b/jenkins/gerrit-bazel-plugin-github.yaml
@@ -6,6 +6,7 @@
       - 'plugin-{name}-gh-bazel-{branch}':
           branch:
             - master
+            - stable-3.13
           extra-artifacts: >
             bazel-bin/plugins/github/github-*.jar,
             bazel-bin/plugins/github/github-*.jar-version