system: Rename activation script.

* gnu/system.scm (operating-system-activation-script): Change file name
  to "activate".

gnu/system.scm

@@ -395,7 +395,7 @@ (define groups
     (define group-specs
       (map user-group->gexp groups))
 
-    (gexp->file "boot"
+    (gexp->file "activate"
                 #~(begin
                     (eval-when (expand load eval)
                       ;; Make sure 'use-modules' below succeeds.