change workflow runner version to ubuntu-latest

Author: bjornregnell

.github/workflows/main.yml

@@ -2,7 +2,7 @@ name: Build
 on: [push]
 jobs:
   Test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: Check out repository code
         uses: actions/checkout@v2