Use ubuntu 20.04 for code coverage

Author: yegappan

.github/workflows/coverage.yml

@@ -3,7 +3,7 @@ on: [push, pull_request]
 jobs:
   linux:
     name: linux
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Install ctags
         run: |