title: Generate unique string in Github Actions
body: <p>I have the following <code>env</code> definition for a job in Github Actions:</p>
  <pre class="lang-yaml prettyprint-override"><code>jobs:
    build-test-deploy:
      runs-on: ubuntu-latest
      env:
        FOO : foobar-${{ github.sha }}
        BAR : BAZ
  </code></pre>
  <p>However the <code>github.sha</code> remains identical when the same workflow is re-run without any commit in between. I need the <code>FOO</code> to be unique for every run (even if it's technically a re-run), independently of commits.</p>
  <p>Two options I'm thinking of:</p>
  <ul>
  <li>either generate a random string</li>
  <li>or get the current unix timestamp</li>
  </ul>
  <p>I don't know how to get either of these values in the context of the <code>env:</code> directive. There are some actions available to use within <code>steps</code> but how is it possible to get this kind of unique values right from the <code>env</code> directive instead?</p>