fix(builder): Non-root slugrunner and slugbuilder

Today apps are build as root and are running as root on deis, but heroku runs them as a non-root user. This removes this difference and fixes missing environment variables in `deis run`.
I think it's worth giving the `slug` user read/write permissions to these folders. In order for that to happen, we need to stop using USER `slug` and revert the behaviour to switch users before running the buildpack compile.