Change the OS used for code coverage runs to Ubuntu 18.04

.github/workflows/coverage.yml

@@ -3,7 +3,7 @@ on: [push, pull_request]
 jobs:
   linux:
     name: linux
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: Checkout Code
         uses: actions/checkout@v2