Addition to #20734, Fixes#20717
The `/index.json` endpoint needs to be accessible even if the registry
is private. The NuGet client uses this endpoint without
authentification.
The old fix only works if the NuGet cli is used with `--source <name>`
but not with `--source <url>/index.json`.
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>