cargo package fails because of `proto` folder

[giarc3]

When running cargo package, there's an error

error: failed to verify package tarball

Caused by:
  Source directory was modified by build.rs during cargo publish. Build scripts should not modify anything outside of OUT_DIR.

This is because our build file outputs to the proto directory. We should fix it to do the correct thing.

For now it can be worked around with --no-verify