Add root nuget.config to ensure CFS protection

nuget.config

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <fallbackPackageFolders>
+    <clear />
+  </fallbackPackageFolders>
+  <packageSources>
+    <clear />
+    <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
+  </packageSources>
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
+</configuration>
+