Fix dnx not authenticating for private feeds

[robertcoltheart]

Fixes #51375

For dnx execution (dotnet tool exec), if the package source is authenticated the previous method of passing an override source meant that the credentials are lost, since this is the equivalient of passing --source https://mysource and ignoring any config in NuGet.config. We need to pass the explicit source for dnx because we confirm with the user that we will be downloading from a particular source (which may be authenticated).

[iamteamstar]

Despite this complexity, congratulations if you can solved the error.

[baronfel]

Thanks for the analysis on the linked issue and this PR - I think it overlaps at least partially with a recently-merged change in #52787 - can you take a look and align your changes with this one to prevent merge conflicts?

[robertcoltheart]

@baronfel Not sure I understand the branch targeting process - the linked PR hasn't been merged into main yet, should I wait for this to be back-merged to main before fixing this PR? Or should I re-target this PR to the release/10.0.3xx branch instead?