CI: install pyyaml so test_lock_creds_before_merge can run

Agent-Logs-Url: https://github.com/githubnext/autoloop/sessions/db0c7c36-cb2d-484b-997e-b523c356f1de

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot

.github/workflows/tests.yml

@@ -13,5 +13,5 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: "3.12"
-      - run: pip install pytest
+      - run: pip install pytest pyyaml
       - run: pytest tests/ -v