Use Python 3.9 on linux workflows

It looks like the dist-packages of python 3.8
on the runner has an older version of ssl package
which is now causing nightly builds to fail.

.github/workflows/linux.yml

@@ -61,6 +61,10 @@ jobs:
           fetch-depth: 2
       - name: Run sccache-cache
         uses: mozilla-actions/sccache-action@v0.0.3
+      - name: Select Python 3.9
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.9'
       - name: Bootstrap Python
         run: python3 -m pip install --upgrade pip virtualenv
       - name: Bootstrap dependencies