guix download: Honor mirrors when using '-o'.

Previously "guix download -o x mirror://gnu/…" would fail.

* guix/scripts/download.scm (download-to-file): Pass #:mirrors to
'url-fetch'.

guix/scripts/download.scm

@@ -49,7 +49,7 @@
       ((or 'file #f)
        (copy-file (uri-path uri) file))
       (_
-       (url-fetch url file)))
+       (url-fetch url file #:mirrors %mirrors)))
     file))
 
 (define* (download-to-store* url #:key (verify-certificate? #t))