Add NuGet config file to otlptestlistener project

[lbussell]

The OTLP test listener test project should use the proper NuGet feed when building, even in a container image.

https://github.com/dotnet/dotnet-docker/tree/nightly/tests/Microsoft.DotNet.Docker.Tests/TestAppArtifacts/otlptestlistener

See #6331 (comment) for more context.