Use .cargo/config.toml instead of .cargo/config

`.cargo/config.toml` is the preferred form for the local configuration
file. This is emphasized in _The Cargo Book_ with the following note:

> Note: Cargo also reads config files without the `.toml` extension, such
> as `.cargo/config`. Support for the `.toml` extension was added in version
> 1.39 and is the preferred form.

Moreover, this helps with toml-aware text editors as they will recognize
the file extension.

Author: samueltardieu