guix archive: Make sure $sysconfdir/guix exists in '--authorize'.

* guix/scripts/archive.scm (authorize-key): Add 'mkdir-p' call.
  Reported by Alex Sassmannshausen <alex.sassmannshausen@gmail.com>.

guix/scripts/archive.scm

@@ -286,6 +286,7 @@ (define (read-key)
 
     ;; Add KEY to the ACL and write that.
     (let ((acl (public-keys->acl (cons key (acl->public-keys acl)))))
+      (mkdir-p (dirname %acl-file))
       (with-atomic-file-output %acl-file
         (lambda (port)
           (display (canonical-sexp->string acl) port))))))