activation: Preserve /etc/groups upon reboots.

This is a followup to e2fcc23a.  The /etc/group file would be cleared
when booting.

* guix/build/activation.scm (activate-users+groups)[touch]: Rewrite so
  it does not wipe out FILE's contents.

guix/build/activation.scm

@@ -93,7 +93,7 @@ (define (activate-users+groups users groups)
 each item in GROUPS is a tuple with the group name, group password or #f, and
 numeric gid or #f."
   (define (touch file)
-    (call-with-output-file file (const #t)))
+    (close-port (open-file file "a0b")))
 
   (define activate-user
     (match-lambda