substitute: Erase the current line when reporting progress.

* guix/scripts/substitute.scm (fetch-narinfos)[update-progress!]: Use
the ANSI erase-current-line sequence next to \r.

guix/scripts/substitute.scm

@@ -613,7 +613,7 @@ if file doesn't exist, and the narinfo otherwise."
     (let ((done 0)
           (total (length paths)))
       (lambda ()
-        (display #\cr (current-error-port))
+        (display "\r\x1b[K" (current-error-port)) ;erase current line
         (force-output (current-error-port))
         (format (current-error-port)
                 (G_ "updating list of substitutes from '~a'... ~5,1f%")