> I think this is a good use case for ‘trivial-build-system’.

Yes, I forgot about it.

>    (let* ((gh   (assoc-ref %build-inputs "source"))

What is the associated value here?  I assume it should be 'gh.pl'.

I've tried to specify (inputs `(("source" ,source))), but it's just a
tarball.  So I'd have to use 'tar' to unpack it (see 'unpack' in
'gnu-build-system.scm').

I guess I'm missing something because you said that "Guile and the (guix
build utils) provide us with everything we need."

Also, why is it necessary to specify (guix build utils) in #:modules and
import it via 'use-modules' in #:builder too?  According to the manual,
all arguments are passed to the build system.