download: Perform derivations locally.

* guix/download.scm (url-fetch): Pass #:local-build? #t to
  'build-expression->derivation'.
* guix/git-download.scm (git-fetch): Likewise.

guix/download.scm

@@ -242,7 +242,11 @@ (define need-gnutls?
                                               (guix build utils)
                                               (guix ftp-client))
                                   #:guile-for-build guile-for-build
-                                  #:env-vars env-vars)))
+                                  #:env-vars env-vars
+
+                                  ;; In general, offloading downloads is not a
+                                  ;; good idea.
+                                  #:local-build? #t)))
 
 (define* (download-to-store store url #:optional (name (basename url))
                             #:key (log (current-error-port)))

guix/git-download.scm

@@ -84,6 +84,7 @@ (define git-for-build
                                   #:recursive? #t
                                   #:modules '((guix build git)
                                               (guix build utils))
-                                  #:guile-for-build guile-for-build)))
+                                  #:guile-for-build guile-for-build
+                                  #:local-build? #t)))
 
 ;;; git-download.scm ends here