dayjs().tz(timezone) uses current timezone offset instead of date timezone offset

**Describe the bug**
`dayjs().tz(timezone)` methods uses Timezone offset at the time of execution. This pose issues when working with Timezone affected by DST.

```js
dayjs('2020-08-01', 'Europe/Paris').format()
// returns 2020-08-01T00:00:00+02:00 all the time
dayjs('2020-08-01', 'Europe/Paris').tz('Europe/Paris').format()
// returns 2020-08-01T00:00:00+02:00 during summer and 2020-08-01T00:00:00+01:00 during winter
```

**Expected behavior**

```js
dayjs('2020-08-01', 'Europe/Paris').tz('Europe/Paris').format()
// should always return 2020-08-01T00:00:00+02:00 because August 1st has a 2h offset (GMT+2) for Europe/Paris
```

**Information**
- Day.js Version 1.9.4
- OS: macOS
- Node.js v12
- Time zone:CET / 'Europe/Paris' (GMT+1)

Start by reproducing the two `dayjs(...).tz('Europe/Paris').format()` examples from the issue with a winter and summer date. Trace the `tz(timezone)` entry point and compare the offset used at execution time with the offset for the represented date; done means August 1 consistently formats with the +02:00 offset.

Written by the indexing model from the issue text.