Change CI ref from merge to head

At this time in the CI, merge is not accessible from tree
https://github.community/t/github-sha-but-how-to-get-the-latest-commit-reference-on-a-branch/17160

.github/workflows/ansible.yml

@@ -7,13 +7,20 @@ jobs:
   # https://molecule.readthedocs.io/en/latest/ci.html#github-actions
   test:
     runs-on: ubuntu-latest
-    # https://docs.github.com/en/free-pro-team@latest/actions/reference/context-and-expression-syntax-for-github-actions#github-context
-    if: github.event_name == 'pull_request' || (github.event_name == 'push' && contains(github.ref, 'refs/heads/'))
     steps:
       - name: Check out the codebase
         uses: actions/checkout@v2
+        if: github.event_name != 'pull_request'
         with:
           path: "${{ github.repository }}"
+      - name: Check out the codebase
+        uses: actions/checkout@v2
+        if: github.event_name == 'pull_request'
+        with:
+          # merge commit is not available from tree at this point in time
+          # https://github.com/actions/checkout#checkout-pull-request-head-commit-instead-of-merge-commit
+          ref: "${{ github.event.pull_request.head.sha }}"
+          path: "${{ github.repository }}"
       - name: Set up Python
         uses: actions/setup-python@v2
       - name: Set up Docker