Reliance on environment variables breaks hermetic builds

[nickpdemarco]

While executing cargo metadata, and perhaps elsewhere, cbindgen invokes the binary specified CARGO environment variable, else an unadorned cargo.

Our build system does not permit reliance on environment variables, and that prevents us from using cbindgen. I'd like to be able to specify --cargo-bin as an argument to cbindgen, and use the following priorities for selecting cargo.

1. --cargo-bin, if specified.
2. CARGO environment variable, if set.
3. cargo, otherwise.

[emilio]

Would happily take a patch for that :)