.NET Framework image builds should not reach out to NuGet.org

[lbussell]

Image builds reach out to NuGet.org to download the roslyn compiler:

dotnet-framework-docker/src/aspnet/4.8/windowsservercore-ltsc2016/Dockerfile

Line 19 in b44a5dd

Invoke-WebRequest https://api.nuget.org/packages/microsoft.net.compilers.2.9.0.nupkg -OutFile C:\microsoft.net.compilers.2.9.0.zip; `

This causes builds to fail when the CFSClean network policy is turned on. It was turned on by default in #1282.

We need to use dnceng-hosted package feeds instead.