Auto merge of rust-lang#7918 - samueltardieu:use-config-toml, r=flip1995

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.

changelog: none

Author: bors