substitute: Update progress for responses different from 200/404.

* guix/scripts/substitute.scm (fetch-narinfos)[handle-narinfo-response]:
Add missing call to 'update-progress!'.

guix/scripts/substitute.scm

@@ -607,10 +607,11 @@ if file doesn't exist, and the narinfo otherwise."
                            #f ttl)
            (update-progress!)
            result))
-        (else                                      ; transient failure
+        (else                                      ; transient failure: 504...
          (if len
              (get-bytevector-n port len)
              (read-to-eof port))
+         (update-progress!)
          result))))
 
   (define (do-fetch uri port)