tonic-build can fail to work if there are artefacts from other build processes

[xd009642]

I have a build script that generates a folder with some resources in the outdir and tonic build tries to run rustfmt on the folder and fails. I've gotten around it by deleting the folder and regenerating the assets after tonic-build is done for every build but I don't think tonic-build should touch anything not generated by it/prost

https://github.com/hyperium/tonic/blob/master/tonic-build/src/lib.rs#L81