Emit build logs for ko,buildpack

[imjasonh]

For failed builds, we can emit logs in the error results

For successful builds the response JSON doesn't include error logs. Instead, we can write the build logs to a file and stuff it into a layer in the resulting image at something like /var/log/build.log, then cache and serve that image. This might break reproducibility, if build logs change at all between two builds, but that's already the case based on ko version, go version, buildpack builder version, etc.