Revert "Use Ubuntu 18.04 build agents"

- too early

This reverts commit 39e9cee.

Author: dougbu

Diff for: .azure/templates/jobs/default-build.yml

@@ -76,7 +76,7 @@ jobs:
     ${{ if and(eq(parameters.poolName, ''), eq(parameters.agentOs, 'macOS')) }}:
       vmImage: macOS-10.14
     ${{ if and(eq(parameters.poolName, ''), eq(parameters.agentOs, 'Linux')) }}:
-      vmImage: ubuntu-18.04
+      vmImage: ubuntu-16.04
     ${{ if and(eq(parameters.poolName, ''), eq(parameters.agentOs, 'Windows')) }}:
       vmImage: vs2017-win2016
       ${{ if ne(variables['System.TeamProject'], 'public') }}: