title: Github Actions: Run step on specific OS
body: <p>I'm running a workflow on some Operating systems.</p>
  <p>However, there is a specific step that I have to run only on Ubuntu:</p>
  <pre><code>runs-on: ${{ matrix.os }}
  strategy:
      matrix:
          os: [ubuntu-latest, windows-latest, macOS-latest]
  steps:
      - name: Setup Ubuntu
        run : export DISPLAY=&quot;127.0.0.1:10.0&quot;
        if: # --&gt; What should be here? &lt;--
  </code></pre>
  <p>I couldn't find any examples nor explanation on how to run steps on specific OSes.</p>
  <p>Can somebody help?</p>