Use Twoliter.override as the source of truth for publication overrides

[cbgbt]

In #372, support was added for allowing pubsys to write to an alternative repository when publishing a kit, yet keeping the kit metadata unchanged.

While discussing the change, we decided that the use of Infra.toml and environment variables as an additional source of truth was a bit confusing. We should use Twoliter.toml and Twoliter.override as the source of truth for kit publication in the future.

[cbgbt]

The other side to this coin is that Infra.toml is used to provide settings for other publications via pubsys. If we do something different here, we should provide a more unified plan for how to configure publications via Twoliter for the artifact types that pubsys supports.