Add `.toml` file extension restriction for `-Zconfig-include` #12298

This is to avoid possible name collisions. For example, a user creates a file called `.cargo/cache`, and then in the future cargo wants to create a directory called `.cargo/cache/`, that would collide with what the user specified. Restricting to `.toml` extensions would avoid that since we won’t make a directory named with a `.toml` extension.