substitute-binary: Don't pretend to report download progress on Guile 2.0.5.

* guix/scripts/substitute-binary.scm (progress-report-port): On Guile
  2.0.5, return PORT directly and emit a warning.

guix/scripts/substitute-binary.scm

@@ -416,9 +416,17 @@ (define (read! bv start count)
       ;; XXX: We're not in control, so we always return anyway.
       n))
 
-  (make-custom-binary-input-port "progress-port-proc"
-                                 read! #f #f
-                                 (cut close-port port)))
+  ;; Since `http-fetch' in Guile 2.0.5 returns all the data once it's done,
+  ;; don't pretend to report any progress in that case.
+  (if (version>? (version) "2.0.5")
+      (make-custom-binary-input-port "progress-port-proc"
+                                     read! #f #f
+                                     (cut close-port port))
+      (begin
+        (format (current-error-port) (_ "Downloading, please wait...~%"))
+        (format (current-error-port)
+                (_ "(Please consider upgrading Guile to get proper progress report.)~%"))
+        port)))
 
 (define %cache-url
   (or (getenv "GUIX_BINARY_SUBSTITUTE_URL")