My proposed patch:

--8<---------------cut here---------------start------------->8---
(add-after 'install 'wrap-program
           (lambda* (#:key inputs outputs #:allow-other-keys)
            (let* ((out  (assoc-ref outputs "out"))
                   (groff (assoc-ref inputs "groff-minimal")))
              (wrap-program (string-append out "/bin/man")
                `("PATH" ":" prefix (,(string-append groff "/bin")))))
            #t))
--8<---------------cut here---------------end--------------->8---

Not completely ideal, but at least man-db would be fully functional for Guix
1.0! :)

-- 
Pierre Neidhardt
https://ambrevar.xyz/