Cannot build a no_std crate that uses a proc macro that uses std

Given:

- A crates index where all crates are no_std compatible
- A crate we want to build for bare metal which uses a proc macro that depends on some other crate that needs std (e.g.: `prost-types`)

rules_rust will pull in the proc macro's dependencies and then propagate the `std` feature to all crates in the index, meaning no we can no longer build our crate for bare metal.