core: Always call log_open() in systemd-executor

core: Always call log_open() in systemd-executor

log_setup() will open the console in systemd-executor because it's
not pid 1 and it's not connected to the journal. So if the log target
is later changed to kmsg, we have to reopen the log.

But since log_open() won't open the same log twice, let's just call it
unconditionally since it will be a noop if we try to reopen the same log.

This makes sure that systemd-executor will log to the log target passed
via --log-target= after parsing arguments.