[timezone] wrong `toISOString()` value with zone conversion and DST

**Describe the bug**

```js
// 'Europe/Vilnius' timezone => GMT+2, DST+1

// zone initialization - ok
dayjs.tz('2022-01-01T00:00:00.000Z', 'Europe/Vilnius').format()
// OUTPUT: '2022-01-01T00:00:00+02:00' ✅
dayjs.tz('2022-01-01T00:00:00.000Z', 'Europe/Vilnius').toISOString()
// OUTPUT: '2021-12-31T22:00:00.000Z' ✅

// zone conversion - fails
dayjs.utc('2021-12-31T22:00:00.000Z').tz('Europe/Vilnius').format()
// OUTPUT: '2022-01-01T00:00:00+02:00' ✅
dayjs.utc('2021-12-31T22:00:00.000Z').tz('Europe/Vilnius').toISOString()
// OUTPUT: '2021-12-31T23:00:00.000Z' ❌
// EXPECTED: '2021-12-31T22:00:00.000Z'
```

FIY: moment.js handles this properly.

**Information**
- Day.js `v1.10.7`
- OS: `macOS Big Sur`

Start by reproducing the timezone conversion with dayjs.utc(...).tz('Europe/Vilnius').toISOString() and compare it with the direct dayjs.tz(...) case shown in the report. Trace the timezone conversion and toISOString() entry points; done means the converted value matches the expected '2021-12-31T22:00:00.000Z' while formatted local output remains correct.

Written by the indexing model from the issue text.