Build readonly plugin for Gerrit v3.12

Change-Id: I24514503b5981225406860d600032fedcd8f33b0
diff --git a/jenkins/gerrit-bazel-plugin-readonly.yaml b/jenkins/gerrit-bazel-plugin-readonly.yaml
index 96c2a2c..e51355d 100644
--- a/jenkins/gerrit-bazel-plugin-readonly.yaml
+++ b/jenkins/gerrit-bazel-plugin-readonly.yaml
@@ -5,6 +5,7 @@
           branch:
             - master
           gerrit-branch:
+            - stable-3.12
             - stable-3.11
             - stable-3.10
       - 'plugin-{name}-bazel-{branch}':