Run PR / Code reviews only on Bazel release

This will improve cache hit.

Change-Id: Ia86d34f1863e5ddb42cc688608b2a2884652a95b
diff --git a/jenkins/jenkins.bzl b/jenkins/jenkins.bzl
index f47653d..3b66114 100644
--- a/jenkins/jenkins.bzl
+++ b/jenkins/jenkins.bzl
@@ -239,6 +239,8 @@
       project_url=project_url,
       platforms=platforms,
       test_platforms=test_platforms)
+  substitutions["BAZEL_VERSIONS"] = "\n".join([
+      v for v in bazel_versions if not v.startswith("HEAD")])
   if pr_enabled:
     jenkins_job(
         name = "PR-" + name,