message: Kill old builds.
diff: +@Library('StanUtils')
  +import org.stan.Utils
  +
  +def utils = new org.stan.Utils()
  +
  def checkout_pr(String repo, String pr) {
  prNumber = pr.tokenize('-').last()
  if (repo == "math") {
  @@ -30,6 +35,14 @@ pipeline {
  description: "Math PR to test against. Will check out this PR in the downstream Math repo.")
  }
  stages {
  + stage('Kill previous builds') {
  + when { not { branch 'develop' } }
  + steps {
  + script {
  + utils.killOldBuilds()
  + }
  + }
  + }
  stage('Clean & Setup') {
  agent any
  steps {
  