This feature request isn't specific to guix build (it was low priority to me as I knew it at least had the ability to log, thanks for telling me it automatically does but it doesn't really change this feature request), it's asking for all guix commands with substantial output to automatically log. I have never run guix build, but I know that some guix commands run for a very long time and if they fail and do not log it, it can be quite irritating to have to re-run it and use tee to log its output. For example, `guix pull && guix package -u` has taken up to 12 hours to run for me before, so if it fails and doesn't log it can be quite an irritating waste of time and deter people from filing invaluable bug reports about issues they've experienced with guix.