progress: Call 'time-difference' on times of the same type.

Guile 2.2.5 and 2.9.2 would catch the issue.
This is a followup to 88bc3c89bf.

* guix/progress.scm (display-download-progress)[elapsed]: Pass
'current-time' the same type as START-TIME.

guix/progress.scm

@@ -197,7 +197,9 @@ object) and TRANSFERRED (a total number of bytes) to determine the
 throughput."
   (define elapsed
     (duration->seconds
-     (time-difference (current-time time-monotonic) start-time)))
+     (time-difference (current-time (time-type start-time))
+                      start-time)))
+
   (if (and (number? size) (not (zero? size)))
       (let* ((%  (* 100.0 (/ transferred size)))
              (throughput (/ transferred elapsed))