SetupNugetSources.ps1 should help out public builds

[mmitche]

• This issue is blocking
• This issue is causing unreasonable pain

Right now SetupNugetSources.ps1 setup private credentials for internal feeds when called. This is all gravy. Unfortunately, internal feeds can sometimes show up in public nuget configs. On release day of an internal build, for instance. Because we can't access these feeds in public dnceng, restores will fail (aside: Nuget covers fixing this in nuget). SetupNugetSources.ps1 should remove these feeds when running on the public projects.

To do this:

1. Change SetupNugetSources.ps1 to take a parameter IsInternalBuild or something along those lines.
2. Change calls to SetupNugetSources.ps1 so that they always run, and pass the appropriate parameter for the internal build flag.
3. If internal, behave as we do today (add feeds)
4. If public, remove any internal feeds.

[mmitche]

Closing in favor of #5890