> I've tried builing locally with the following command, and I get
something that suggests there may be a problem, but I'm not sure where
to go from here.
>
> $ ./pre-inst-env guix build --no-grafts --rounds=2 --check ruby
>
> guix build: error: build failed: derivation `/gnu/store/37rb11ivy4w6ydn0px386k158ixbgczz-ruby-2.5.1.drv' may not be deterministic

Not sure if I am the best person to ask, but I generally build it once, copy the store directory to /tmp, delete it from the store with "gc", then build it again. Finally, compare the two directory trees - I use "meld" (don't think this is in Guix, unfortunately), but others use "diffoscope", as I understand.

HTH, ben