feat(arch-nspawn): use --keep-unit when running inside a service

The --keep-unit argument for nspawn was removed in 5f4fd52e. I didn't realize it was a workaround until then. This commit brings it back when devtools is running inside a service, as the message in that commit says:

`--keep-unit` is really only for use in services like `systemd-nspawn@.service`.

And yes, my program runs it inside services (using systemd-run) for resource monitoring. The new naming scheme isn't predictable so my program can't track there (and even if it could, I may still need a way to pass some systemd/cgroups options to that thing).