Add a nuget.config that can be used all the time. (#3376)

* Stop trying to be cute, just let the nuget.config live on

* Revert changes to workflow

Author: IEvangelist

.github/workflows/snippets5001-release.yml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <!-- Try the dotnet9 feed first, if we can't find our packages there, fallback to the nuget feed. -->
+    <add key="dotnet9" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet9/nuget/v3/index.json" />
+    <add key="nuget" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+</configuration>