gnu: bootstrap: Add FTP URL for bootstrap binaries.

* gnu/packages/bootstrap.scm (%bootstrap-base-urls): Add
ftp://alpha.gnu.org (http://alpha.gnu.org is currently unresponsive.)

gnu/packages/bootstrap.scm

@@ -287,7 +287,8 @@ (define %bootstrap-guile
 
 (define %bootstrap-base-urls
   ;; This is where the initial binaries come from.
-  '("http://alpha.gnu.org/gnu/guix/bootstrap"
+  '("ftp://alpha.gnu.org/gnu/guix/bootstrap"
+    "http://alpha.gnu.org/gnu/guix/bootstrap"
     "http://www.fdn.fr/~lcourtes/software/guix/packages"))
 
 (define %bootstrap-coreutils&co