collab: Override Cargo configuration in the `Dockerfile` (#17901)

This PR moves the override for the Cargo configuration for collab into
the `Dockerfile` rather than having it be something some in the external
environment.

This makes it possible to build the Docker image locally without having
to replace `.cargo/config.toml` with the contents of
`.cargo/collab-config.toml`.

Release Notes:

- N/A