title: Concourse CI: Use Metadata (Build number, URL etc) in on_success/on_failure
body: <p>How is it possible to use <a href="https://github.com/pivotal-cf/email-resource" rel="nofollow noreferrer">Metadata</a> in <code>on_success</code>/<code>on_failure</code>? For example, to send emails via <a href="https://github.com/pivotal-cf/email-resource" rel="nofollow noreferrer">https://github.com/pivotal-cf/email-resource</a>?</p>
  
  <p>I haven't found a way, as I can't change content of files where email resources reside (subject/body), as the metadata is not available to tasks.</p>
  
  <p>And yep, that might be a duplicate for <a href="https://stackoverflow.com/questions/39771347/concourse-ci-and-build-number">Concourse CI and Build number</a></p>
  
  <p>But still my question IMHO is a valid use case for notifications.</p>