Add allow list for crates with build.rs

[repi]

We talked about adding support for an explicit list of crates that we allow having build.rs build step.

build.rs is not sandboxed so it usually has full access to the developer machine it is running on which is a potential exploit surface + we generally want to keep custom build steps and C/C++ dependencies to a mininum to reduce complexity and improve build times.

Related to EmbarkStudios/rust-ecosystem#17