Update Build Image (#115)

***NO_CI***

Author: networkfusion

azure-pipelines.yml

@@ -22,7 +22,7 @@ jobs:
 - job: Build_mscorlib
   condition: or( eq(variables['UPDATE_DEPENDENTS'], 'false'), eq(variables['StartReleaseCandidate'], 'true') )
   pool:
-    vmImage: 'VS2017-Win2016'
+    vmImage: 'windows-2019'
 
   variables:
     DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true