housekeeping: add a nuget.config file

nuget.config

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+DO NOT REMOVE THIS FILE
+There is an issue where the nuget config local cache can play up.
+Already had similar issues with the local cache and it containing netcore 3.1 packages.
+https://github.com/actions/virtual-environments/issues/1090
+-->
+<configuration>
+  <packageSources>
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
+  </packageSources>
+</configuration>