Build gc-conductor plugin for stable-3.1

Change-Id: I17aad5815e2dee32e4fd9a798c88c709edce01eb
diff --git a/jenkins/gerrit-bazel-plugin-gc-conductor.yaml b/jenkins/gerrit-bazel-plugin-gc-conductor.yaml
index fe48dca..18afa48 100644
--- a/jenkins/gerrit-bazel-plugin-gc-conductor.yaml
+++ b/jenkins/gerrit-bazel-plugin-gc-conductor.yaml
@@ -9,5 +9,6 @@
             - stable-3.2
       - 'plugin-{name}-bazel-{branch}':
           branch:
+            - stable-3.1
             - stable-3.0
             - stable-2.16