Build project-download-commands plugin on stable-3.1 branch

Change-Id: I8610224c56c83e704fb432b10beccce51c78139f
diff --git a/jenkins/gerrit-bazel-plugin-project-download-commands.yaml b/jenkins/gerrit-bazel-plugin-project-download-commands.yaml
index 64b47da..b652068 100644
--- a/jenkins/gerrit-bazel-plugin-project-download-commands.yaml
+++ b/jenkins/gerrit-bazel-plugin-project-download-commands.yaml
@@ -4,3 +4,4 @@
       - 'plugin-{name}-bazel-{branch}':
           branch:
             - master
+            - stable-3.1