mkosi: properly remove root tree and /var/tmp between first and second build
mkosi: properly remove root tree and /var/tmp between first and second build

Let's make sure to remove both the old OS tree and its /var/tmp file
after the first build finished and we ran the build script inside of it,
but before we start the next iteration.

This fixes "-t directory" builds.

UbiCastTeam/mkosi,UbiCastTeam/mkosi,systemd/mkosi,systemd/mkosi