vmspawn: Support RUNTIME_DIRECTORY again (#41619)

In ccecae0efd ("vmspawn: use machine name in runtime directory path")
support for RUNTIME_DIRECTORY was dropped which makes it difficult to
run systemd-vmspawn in a service unit which doesn't have write access to
the regular /run but should use its own managed RUNTIME_DIRECTORY. What
worked before was --keep-unit --system but we can't use XDG_RUNTIME_DIR
and --user because then --keep-unit breaks which we need because it
can't create a scope as there is no session. Switch back to
runtime_directory which handles RUNTIME_DIRECTORY and tells us whether
we should use it as is without later cleanup or if we need to use the
regular path where we create and delete the directory ourselves.