eib_image: Force flush memory buffers to storage after rsync

https://app.asana.com/1/1203676861188277/project/1211949321657103/task/1212760656967809

Author: starnight

stages/eib_image

@@ -311,6 +311,7 @@ create_image() {
   # copying, and its use in combination with -H is documented to do what we
   # want.
   rsync -aHAX --no-inc-recursive "${EIB_OSTREE_CHECKOUT}/" "${ROOT}/"
+  sync
 
   eib_write_version_xattr "${ROOT}"