Fix postgres command checking

Author: ldionne

.github/workflows/tox.yaml

@@ -34,7 +34,7 @@ jobs:
         postgresql-version: 17
     - name: Ensure PostgreSQL is not installed
       if: ${{ ! matrix.with-postgres }}
-      run: ! postgres
+      run: ! which postgres
     - name: Tox flake8
       run: tox -e flake8
     - name: Tox docs