rename the build.rs such that it's not run by default

[zancas]

This script is solely for the purpose of producing Rust source code from protobuf spec files, to support lightwalletd gRPC services.

It should only be run when a developer explicitly intends to change [add/remove/modify] those bindings.

It's unlikely this will be the case for the majority of builds, and probably shouldn't be done implicitly.