Is there a way to upgrade only packages with substitutes?

[Katherine Cox-Buday]

Hey Guix!

Very often I find myself updating systems only to get stuck on building
something large like LibreOffice or Un-Googled Chromium. Usually the
substitutes for these are available a few days later.

It would be nice if I could pass a flag to `guix package -u` that would
disallow building in the event a substitute wasn't available. Is that
`--fallback=false`? If so, that's a double-negative; is there a clearer
way to represent both without acreting options?

-- 
Katherine