title: Jenkinsfile - How to detect destination of pull request
body: <p>so I have a <code>jenkinsfile</code> where I have my pipeline. I would like to be able to distinguish between a pull request which is going to the master and pull requests going elsewhere. I have found example code for detecting pull requests - something like this:</p>
  
  <pre><code>env.BRANCH_NAME.startsWith('PR-')
  </code></pre>
  
  <p>But is there any way to find out the target of the pull request from some <code>env.</code> variable?</p>