toISOString() is inconsistent with set timezone

Running
```
const dayjsObj = dayjs('2020-01-02T15:14:59.741774+00:00').tz('America/New_York', true);
console.log(dayjsObj.toISOString());
```
gives inconsistent results depending on the timezone the computer is running it in. For example, a computer in the `America/New_York` timezone would log out `2020-01-02T15:14:59.741Z` whereas a computer in `UTC` would log out `2020-01-02T20:14:59.741Z`.

I believe this is due to the following line of code:

https://github.com/iamkun/dayjs/blob/a9d7d0398d22ebd4bfc3812ca0134a97606d54d9/src/plugin/utc/index.js#L117

For the computer in `America/New_York`, running `new Date().getTimezoneOffset()` gives `300` and for the computer in `UTC`, it would give `0`. In both computers, `dayjsObj.$offset` is `-300` and there is no `dayjsObj.$x.$localOffset`. As a result, `addedOffset` would be `0` for the computer in `America/New_York` but `-300` for the computer in `UTC`. This `addedOffset` would be used to create a new `Date` object in which the ISO string is derived from.

Is this intended behavior or should the `dayjsObj` provide the same ISO string regardless of the timezone a computer is running in?

Start with the reproduction in the issue and inspect src/plugin/utc/index.js at the linked line, focusing on how addedOffset is calculated from $offset and the host timezone. Compare toISOString() output under America/New_York and UTC, then determine and document the intended consistent behavior and the verification needed for it.

Written by the indexing model from the issue text.