guest-overlay: Always recreate squashfs without appending

Without `-noappend`, we end up with an image containing `gast`, `gast_1`,
`gast_2` etc. after running repeatedly.  This way, it is always recreated and
contains only what it is supposed to.

guest-overlay/tasks/main.yml

@@ -43,6 +43,7 @@
       - 1000
       - -force-gid
       - 1000
+      - -noappend
   delegate_to: localhost
   run_once: true
   tags: