download: Provide a fall-back guile for older daemons.

Fixes <http://bugs.gnu.org/25775>.

* guix/download.scm (in-band-download): Use %bootstrap-guile as a
fall-back when using older daemons that do not have a built-in
downloader.

Written-by: Ricardo Wurmus <rekado@elephly.net>

guix/download.scm

@@ -418,7 +418,10 @@ GnuTLS itself and its dependencies.  See <http://bugs.gnu.org/22774>."
                        ;; hash of the expected result.
                        #:verify-certificate? #f)))))
 
-  (mlet %store-monad ((guile (package->derivation guile system)))
+  (mlet %store-monad ((guile (package->derivation
+                              (or guile
+                                  (@@ (gnu packages bootstrap) %bootstrap-guile))
+                              system)))
     (gexp->derivation file-name builder
                       #:guile-for-build guile
                       #:system system