Update nuget.config to use CFS feed (#1649)

Author: adityapatwardhan

nuget.config

@@ -2,7 +2,7 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+    <add key="PowerShellCore_PublicPackages" value="https://pkgs.dev.azure.com/powershell/PowerShell/_packaging/powershell/nuget/v3/index.json" />
   </packageSources>
   <disabledPackageSources>
     <clear />