Fix test runner to ubuntu-20.04

.github/workflows/run_tests.yml

@@ -1,4 +1,4 @@
-name: Run tests
+name: Run Python unit tests
 
 on:
   push:
@@ -10,7 +10,7 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v2