Automatically trim down smoke-testNuGet.Config for tarball, instead of having extra copy at support/tarball/smoke-testNuGet.Config

[dagood]

source-build/eng/SourceBuild.Tarball.targets

Lines 154 to 164 in 0559784

	<!--
	When building an internal build, the main smoke-test nuget config has authenticated sources
	in it to generate a full set of smoke-test-prereqs. This isn't necessary in the tarball
	because we have those smoke-test-prereqs, but we hit authentication errors nonetheless. Copy
	a trimmed down nuget.config in this case.
	TODO: Automatically trim down the main nuget.config.
	-->
	<TarballCopyFile
	Condition="Exists('$(ProjectDir)support\tarball\smoke-testNuGet.Config')"
	Include="$(ProjectDir)support\tarball\smoke-testNuGet.Config"
	RelativeDestination="smoke-testNuGet.Config" />

We have targets that manipulate nuget.configs in exactly the way we need, just need to apply them in the tarball generation targets. This fixes some minor code (nuget.config) duplication.

[omajid]

This should have been closed by #2159