ci: Switch runs-on to ubuntu-20.04

The DockerFile being used can stay at 18.04

.github/workflows/test.yml

@@ -2,7 +2,7 @@ name: test
 on: [push]
 jobs:
   unittests:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v2