title: Pass checkout info in a github actions script
body: <p>I have a self-hosted github runner that launches python script.  This python script runs a bash shell script on a different machine, and I think I can pass arguments to either script.  Right now, I have set it up to clone the <code>main</code> branch of my repo, or I can pass it an argument to check out a different branch.  This works great for testing when tags are issued.</p>
  <p>I want to set this up so that it will test the code in a <code>pull_request</code>.  The <code>pull_request</code>s come from forks of the repository, not branches within the repository.  My understanding is that the SHA won't be available in the repo because it only exists on the fork, so I <em>can't</em> do something like</p>
  <pre><code>git clone &lt;URL&gt;.FMS.git &amp;&amp; cd FMS &amp;&amp; git merge $GITHUB_SHA
  </code></pre>
  <p><strong>How do I pass the information from the fork and fork's branch/SHA to merge into the <code>main</code> branch in the repository?</strong> I can then set my script to do (?)</p>
  <pre><code>git clone &lt;URL&gt;/FMS.git &amp;&amp; cd FMS &amp;&amp; git pull &lt;FORK-URL&gt;/FMS.git $GITHUB_SHA
  
  </code></pre>
  <p>Here is how I have my workflow currently set up</p>
  <pre class="lang-yaml prettyprint-override"><code>on: pull_request
  jobs:
     pw:
      runs-on: [self-hosted, pwp]
      strategy:
        fail-fast: false
        max-parallel: 2
        matrix:
          include:
                  - runname: FMS with intel 18
                    runscript: python3 /home/pw/storage/pw_api_python/FMStestStartClusters.py cluster
                  - runname: FMS with intel 2021 container
                    runscript: python3 /home/pw/storage/pw_api_python/FMSintel21StartClusters.py cluster_two
      steps:
                  - name: FMS make check
                    env:
                            RUNNAME: ${{ matrix.runname }}
                            RUNSCRIPT: ${{ matrix.runscript }}
                    run: $RUNSCRIPT
  </code></pre>