The `--wait` flag for `systemd-run` was added in systemd 232. While it
is quite old, it doesn't hurt to special-case them and skip the `--wait`
flag. The consequence is that we lose the update command output in logs,
but at least auto-updates will work.
Fixes#12136
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>