title: github actions exclude pull requests from a branch
body: <p>Say I have a workflow that runs on every PR to master which starts with:</p>
  <pre class="lang-yaml prettyprint-override"><code>on:
    pull_request:
      branches:
        - master
  </code></pre>
  <p>I'd like to skip all jobs if the PR is coming from the <code>depbot</code> branch. Something like:</p>
  <pre class="lang-yaml prettyprint-override"><code>on:
    pull_request:
      branches:
        - master
      head_ref-ignore:
        - depbot
  
  </code></pre>
  <p>I think you can skip all the steps (one at a time) using</p>
  <pre class="lang-yaml prettyprint-override"><code> if: startsWith(github.head_ref, 'depbot') == false
  </code></pre>
  <p>But it is not what I want, as it would still initiate the job. How can I achieve that at the launch level?</p>