Enforce use of azure feed in test baseline nuget.config. (#5559)

Author: imcarolwang

src/dotnet-svcutil/lib/tests/Baselines/TFMBootstrap/tfmDefault/nuget.config

@@ -2,6 +2,6 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key = "svcutilTestFeed" value="$repositoryRoot$/artifacts/packages/RelType/ShipType" />
+    <add key = "dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
   </packageSources>
 </configuration>