Add a --cargo-bin flag to choose the cargo binary

[ChrisJr404]

This adds a --cargo-bin flag so cbindgen can be pointed at a specific cargo binary for the cargo metadata and cargo expand calls, as asked for in #1049. It takes precedence over the CARGO environment variable, which still falls back to a bare cargo on PATH, so the default behavior is unchanged.

This is handy for hermetic build systems that don't allow relying on environment variables. There's also a with_cargo_bin method on the builder for the library API, alongside the existing with_lockfile.