Implement build overlay mounting with mkosi-sandbox

Implement build overlay mounting with mkosi-sandbox

Now that we have Context.rootoptions(), we can switch out how we set
up the root mount without having to modify code all over the place.

Let's use this to get rid of mount_build_overlay() and instead replace
it with setup_build_overlay(), which simply configures a bunch of
fields on Context that make rootoptions() set up the root mount as an
overlay instead of a bind mount.