`cargo upgrade` doesn't work with local registries

When using a local registry, I get:

```
❯ cargo upgrade
Error: Invalid cargo config
```

Start by reproducing `cargo upgrade` with a local registry and tracing how the command reads or validates Cargo configuration. The issue provides no file or test entry point; done means local-registry configuration is accepted and dependency upgrading works without the invalid-config error.

Written by the indexing model from the issue text.