Add internal build support (dotnet/core-setup#8279)

Add internal build support (dotnet/core-setup#8279)

Add NuGetAuthenticate task.

Create proxy build scripts that include pre-build auth plugin install.

Add workaround to avoid running test restore against authenticated feed.

Pass the NuGet auth env vars set by the AzDO build step into the Docker container.

Add configurability for internal publish.

Commit migrated from https://github.com/dotnet/core-setup/commit/a896b3ee381b6d58830613def6f118e16a8a6dc7