Update tests.yml

Fix build failed based on the guideline: actions/setup-python#544
The issue seems to simply be that there is no available build of Python 3.6 from ubuntu-22.04

Author: thanhtung09t2

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ permissions:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: ['3.6', '3.7', '3.8', '3.9', '3.10']