[CI][VSTS] Re-enable the nuget feed push for the preview. (#10551)

* [CI][VSTS] Publish nugets to the preview feed.

fixes: https://github.com/xamarin/xamarin-macios/issues/10300

* Be verbose when publishing to know what is really going on.

* Make bash verbose too.

* Use the yaml step as Xamarin.Android does.

* We do not longer use the bash script.

Co-authored-by: Rolf Bjarne Kvinge <rolf@xamarin.com>

tools/devops/automation/templates/build/build.yml

@@ -308,6 +308,15 @@ steps:
   continueOnError: true # should not stop the build since is not official just yet.
   timeoutInMinutes: 180
 
+- task: NuGetCommand@2
+  displayName: 'Publish Nugets'
+  inputs:
+    command: push
+    packagesToPush: $(Build.SourcesDirectory)/package/*.nupkg
+    nuGetFeedType: external
+    publishFeedCredentials: xamarin-impl public feed
+  condition: and(succeeded(), eq(variables['configuration.BuildNugets'], 'True'))
+
 - bash: $(Build.SourcesDirectory)/xamarin-macios/tools/devops/automation/scripts/bash/productsign.sh
   env:
     PRODUCTSIGN_KEYCHAIN_PASSWORD: $(xma-password)