import: gnu: Propagate the key-download argument.

* guix/import/gnu.scm (gnu->guix-package): Pass the key-download
  argument on to gnu-package->sexp.

guix/import/gnu.scm

@@ -102,7 +102,7 @@ (define* (gnu->guix-package name
      (let ((version (gnu-release-version release)))
        (match (find-packages (regexp-quote name))
          ((info . _)
-          (gnu-package->sexp info release))
+          (gnu-package->sexp info release #:key-download key-download))
          (()
           (raise (condition
                   (&message