Go back to older SetupNuGetSources.ps1

[mmitche]

The new version of SetupNuGetSources.ps1 and associated templates should work equivalently to the old script, but appears to have odd behavior in some repos. It doens't seem to work at all in aspnetcore, and SDK has a few scenarios where using env vars for the creds instead of the cleartext cred does not work. Furthermore, some repos may rely on the nguet.config containing creds for some test scenarios.

This PR simplifies the cred transition for now.

• Revert to the old script, but allow for no cred to be passed
• Add legacy cred support to the new templates.

To double check:

• [ ] The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[mmitche]

Still need some verification here.

[ViktorHofer]

Note that roslyn and runtime's official builds are also failing.

[mmitche]

Note that roslyn and runtime's official builds are also failing.

Those are unrelated. This are only an issue in repos that use SetupNuGetSources