Pin Python version to 3.8 in release validation step.

* Matches the pinned version elsewhere.
* Without this, was picking up 3.10, which we don't build yet.

.github/workflows/validate_and_publish_release.yml

@@ -37,7 +37,7 @@ jobs:
         id: set_up_python
         uses: actions/setup-python@v2
         with:
-          python-version: '3.x'
+          python-version: '3.8'
       - name: Install python packages
         id: install_python_packages
         run: |