Add feed authentication in preparation for moving to CFS (Azure#52790)

Contributes to Azure/azure-sdk-tools#9101

Author: weshaggard

eng/pipelines/templates/steps/install-dotnet.yml

@@ -4,6 +4,9 @@ parameters:
   Container: false
 
 steps:
+  # Need to authenticate so our pipeline can update packages in the feed.
+  - task: NuGetAuthenticate@1
+
   # Installation steps need to be uncommented when switching to a newer SDK that's not available on DevOps agents
   - task: UseDotNet@2
     displayName: 'Use .NET SDK'