pkgupdate: fix missing ROOT_DIR in postupdate hook on replan

The previous expectation that ROOT_DIR is already set is true only if current execution is not reexecution (subsequent execution after replan). In case of reexecution the ROOT_DIR set is skipped so we have to add it for postinst as well.

That applies for both postupdate as well as reboot required hook.

Fixes #309 (closed) and turris/turris-os-packages#678 (well all those hooks check if ROOT_DIR is / so if that variable is empty the postupdate is just skipped).