Verify that Postgres is not installed unless requested

Author: ldionne

.github/workflows/tox.yaml

@@ -32,6 +32,9 @@ jobs:
       uses: tj-actions/install-postgresql@v3
       with:
         postgresql-version: 17
+    - name: Ensure PostgreSQL is not installed
+      if: ! ${{ matrix.with-postgres }}
+      run: ! postgres
     - name: Tox flake8
       run: tox -e flake8
     - name: Tox docs