nspawn: make sure to chown() implicit source dirs for --bind= to container root UID

nspawn: make sure to chown() implicit source dirs for --bind= to container root UID

This makes sure that a switch like --bind=:/foo does the right thing if
user namespacing is one: the backing dir should be owned by the
container's root UID not the host's. Thus, whenever the source path is
left empty and we automatically generate a source dir as temporary
directory, ensure it's owned by the right UID.

Fixes: #20869