Use Ubuntu 20.04 for Linux build.

actions/runner-images#6002

.github/workflows/main.yml

@@ -3,7 +3,7 @@ on: [push, pull_request]
 
 jobs:
   linux-os:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - name: Update APT
       run: sudo apt-get update