[Darc/Maestro++] Default channels are ignored if commit is not accessible on GitHub without authentication

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

Symbol publishing from the internal build does not seem to work when the github repo is private. According to our chat, this is because the latest commit (from internal repo) is attempted to be pulled from github (since the commits are just mirrored), but it's done in an unauthenticated way, which means private repos don't work.

The workaround is to just add the internal azdo repo in `darc add-default-channel` instead of the github repo.