Use Python 3.8 in job checking action

This helps detect the use of unsupported language features.
Unfortunately there's no easy way to detect use of unsupported library
features.

Other tasks stay on the latest Python version.

Signed-off-by: Tu Dinh <ngoc-tu.dinh@vates.tech>

.github/workflows/jobs-check.yml

@@ -9,6 +9,8 @@ jobs:
     - uses: actions/checkout@v2
     - name: Set up Python
       uses: actions/setup-python@v4
+      with:
+        python-version: 3.8
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip