Dayjs utcOffset set twice will get incorrect utc offset.

**Describe the bug**
When I set the utcOffset at the second time, the time will be change to my local timezone.

**Expected behavior**
Timezone should be changed according to the setting


My current timezone is +8
When I set a dayjs object with +7, it is working well.
2021-10-23 09:00 +7 is the expected result.
Then I want to have a +0 result, which ought to be 2021-10-23 02:00 +0
But I got '2021-10-23T10:00:00Z'

In moment.js , it is fine.
```
moment.utc("2021-10-23 09:00").utcOffset(7, true).utcOffset(0).format()
// '2021-10-23T02:00:00Z' -- ok
```

I think the 10AM came from 2+8 since my local timezone is +8

By now, how I solve it is formatting the object to string, when convert the string to object:
```
const a = dayjs.utc("2021-10-23 09:00").utcOffset(7, true).format()
// '2021-10-23T09:00:00+07:00'
const b = dayjs.utc(a).format()
// '2021-10-23T02:00:00Z' -- ok
```

**Information**
- Day.js Version [v1.10.4]
- OS: [iOS]
- Browser [chrome Version 93.0.4577.82 (Official Build) (x86_64)]
- Time zone: [GMT+0800 (China Standard Time)]

Thanks for your time!

Start with the utcOffset call sequence in the reproduction and trace how the second offset is applied after utcOffset(7, true). Use the expected 2021-10-23T02:00:00Z result as the regression case, and verify that repeated offset changes preserve the intended instant and offset behavior.

Written by the indexing model from the issue text.