title: How to allow scripts to access OAuth token from yaml builds
body: <p>My build script uses the <code>SYSTEM_ACCESSTOKEN</code> environment variable.</p>
  
  <p>In the designer build definition I checked <em>Allow scripts to access the OAuth token</em> and everything works.</p>
  
  <p>After copying the designer generated YAML definition I cannot access the <code>SYSTEM_ACCESSTOKEN</code> environment variable.</p>
  
  <p>How do I allow my YAML build to access the OAuth Token?</p>
  
  <p>This is my azure-pipelines.yaml:</p>
  
  <pre><code>queue:
    name: Hosted VS2017
  
  steps:
  - checkout: self
    lfs: true
    persistCredentials: true
  
  - powershell: ./build.ps1
  </code></pre>