add post-refresh hook to restart subiquity on the serial line
add post-refresh hook to restart subiquity on the serial line

For https://bugs.launchpad.net/subiquity/+bug/1848182.

This was easy? As a bonus, the hook from the refreshed-to snap is
executed so this will work even when the user is running current media.

CanonicalLtd/subiquity,CanonicalLtd/subiquity