When `tmpl` substitutes the path of the generated file for `%s` in hook
commands, it uses the full path including the `destdir` prefix. Since
we're running `tmpl` inside a container, but `systemd-sysusers` outside
it (via `nsenter -t 1`), that path is not correct. Thus, we need to
explicitly pass the path as `systemd-sysusers` will see it.