fix: offset logic calculated for east_opt not for west_opt

[rlienlaf]

While testing Job::new_async_tz on different timezones with specific set hours next_tick_for_job() was giving wrong results. On further inspection, the next_tick for every job with given timezones was being calculated wrong because the offset logic wasn't working properly while using west_opt() when calculating the fixed offset, mainly because of how signs are treated on FixedOffset. Changing west_opt() for east_opt() should suffice as a quick fix. Also, as an alternative solution, when initially calculating time_offset_seconds in JobLocked::new_async_tz, instead of local_minus_utc(), utc_minus_local() can be used in conjunction with the original west_opt().

As an example to replicate this problem, following the README, a job can be created like this

    let mut job = Job::new_async_tz("1/7 * 17 * * *", Santiago, |uuid, mut l| {
        Box::pin(async move {
            println!("I run async every 7 seconds");

            // Query the next execution time for this job
            let next_tick = l.next_tick_for_job(uuid).await;
            match next_tick {
                Ok(Some(ts)) => println!("Next time for 7s job is {:?}", ts),
                _ => println!("Could not get next tick for 7s job"),
            }
        })
    })?;

Pay attention to the value returned by next_tick_for_job().

[dgonzf1]

Gran trabajo maquina