(define* (unpack #:key source #:allow-other-keys)
  "Unpack the jar archive SOURCE.  When SOURCE is not a jar archive fall back
to the default GNU unpack strategy."
  (if (string-suffix? ".jar" source)
      (begin
        (mkdir "src")
        (with-directory-excursion "src"
          (invoke "jar" "-xf" source))
        #t)
      ;; Use GNU unpack strategy for things that aren't jar archives.
      ((assq-ref gnu:%standard-phases 'unpack) #:source source)))

Hmm... maybe the patched source gets a random name that doesn't end in ".jar"