Since systemd 254, `systemd-run --scope` expands $VAR / ${VAR} / $$ in the
command line itself (--expand-environment defaults to yes), so a background
command under a supervised gateway ran rewritten: `$$` became a literal `$`,
`${arr[@]}` became '' with a warning on stderr.
_systemd_scope_argv is the single argv choke point shared by the availability
probe, the pipe/PTY background spawns and restart-safe cron/kanban children,
so pass `--expand-environment=no` there. systemd-run < 254 rejects the
option (and never expanded in scope mode), so the existing cached probe drops
it and re-probes once when systemd-run names it in the rejection — no extra
subprocess on modern hosts and no lost isolation on old ones.
Fixes#132385.
9cf7960f27