title: Jenkins pipeline milestone not cancelling previous ongoing build
body: <p>I am experimenting with Jenkins pipeline and milestones and cannot figure out why Jenkins is not cancelling the previous build when a new build crosses the milestone.</p>
  
  <p>Example Jenkinsfile</p>
  
  <pre><code>pipeline {
      agent any
  
      parameters {
          booleanParam(defaultValue: true, description: '', name: 'userFlag')
      }
  
      stages {
          stage("foo") {
              steps {
                  milestone(ordinal: 1, label: "BUILD_START_MILESTONE")
                  sh 'sleep 1000'
              }
          }
      } 
  }
  </code></pre>
  
  <p>Triggering this pipeline twice does not cancel the 1st job</p>