Use default NuGet configuration for scenario tests

[lbussell]

This is the preferred solution to #5704. In order to more accurately validate user experience, we should use the default NuGet configuration for scenario tests in both main and nightly (except for pre-release versions of .NET, which require a special NuGet feed).

The relevant NuGet configs that we use today are here:

• nightly: https://github.com/dotnet/dotnet-docker/blob/5492718d4ddb6949b3853aac148b784a6572c8fe/tests/Microsoft.DotNet.Docker.Tests/TestAppArtifacts/NuGet.config.nightly
• main: https://github.com/dotnet/dotnet-docker/blob/5492718d4ddb6949b3853aac148b784a6572c8fe/tests/Microsoft.DotNet.Docker.Tests/TestAppArtifacts/NuGet.config

Once this is complete, the workaround from #5786 should be removed.

[lbussell]

[Triage] We should continue to use the internal mirror feeds for the nightly repo.

We should also make sure to clean up the feeds for the servicing versions of .NET in the nightly branch, since we are no longer testing pre-release versions of those .NET versions in the nightly branch.

dotnet-docker/tests/Microsoft.DotNet.Docker.Tests/TestAppArtifacts/NuGet.config.nightly

Lines 5 to 6 in 5492718

	<add key="dotnet6" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json" />
	<add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />