Switch Github runners to Ubuntu 20.04 (#1222)

Author: aidanharan

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ on:
 jobs:
   test:
     name: Run test suite
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04 # TODO: Change back to 'ubuntu-latest' when https://github.com/microsoft/mssql-docker/issues/899 resolved.
 
     env:
       COMPOSE_FILE: docker-compose.ci.yml