activation: Only create groups that do not exist yet.

Before that the effect would be to re-create groups at each boot, and
thus remove any members of the groups.

* guix/build/activation.scm (activate-users+groups): Call 'add-group'
  only when (getgrname name) fails.

guix/build/activation.scm

@@ -126,7 +126,8 @@ (define activate-user
   ;; Then create the groups.
   (for-each (match-lambda
              ((name password gid)
-              (add-group name #:gid gid #:password password)))
+              (unless (false-if-exception (getgrnam name))
+                (add-group name #:gid gid #:password password))))
             groups)
 
   ;; Finally create the other user accounts.