Follow-up for 28459ba1f4
The pty path returned by OpenMachinePTY() cannot be opened from outside
the machine, hence let's use the plain Standard{Input,Output,Error}=tty
in such a case. This means if --machine= is specified, #32916 would occur.
A comprehensive fix requires a new dbus method in machined, which shall
be material for v257.
See also: https://github.com/systemd/systemd/pull/33216#discussion_r1628020429
Replaces #33216
Co-authored-by: Mike Yuan <me@yhndnzj.com>
ddef3ec87c