nut: Pass explicit path to systemd-sysusers

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.

pkg/nut/templates.k

@@ -4,7 +4,7 @@ templates = [
         dest = "/etc/sysusers.d/nut.conf"
         hooks = {
             changed = [{
-                run = "systemd-sysusers %s"
+                run = "systemd-sysusers /etc/sysusers.d/nut.conf"
                 immediate = True
             }]
         }