DO SDK: Specify windows-2019 build VM image

 * Default image has moved to windows-2022 which has VS2022 installed by default instead of
   VS2019. We have not yet tested builds with VS2022, so go back to windows-2019 for now.
   See actions/runner-images#4856.

azure-pipelines/build/windows/dosdkcpp.yml

@@ -32,7 +32,7 @@ pr:
       - sdk-cpp/build/cleanup-install.sh
 
 pool:
-  vmImage: "windows-latest"
+  vmImage: "windows-2019"
 
 jobs:
 - job: Debug