utcOffset() does not respect .tz()

Running
```
const dayjsObj = dayjs
.utc('2020/12/30 12:00', 'YYYY/MM/DD hh:mm', true)
.tz('America/New_York');
const offset = dayjsObj.utcOffset();
console.log({
dayjsObj,
'dayjsObj.format()': dayjsObj.format(),
offset,
dayjsObj.utcOffset(offset)': dayjsObj.utcOffset(offset),
'dayjsObj.utcOffset(offset).format()': dayjsObj
.utcOffset(offset)
.format(),
'dayjsObj.local().format()': dayjsObj.local().format(),
'dayjsObj.local()': dayjsObj.local(),
});
```
gives inconsistent output. On a computer where `dayjsObj.local().format()` is `2020-12-30T12:00:00+00:00`, `dayjsObj.utcOffset(offset).format()` gives `2020-12-30T12:00:00-05:00`:
```
{
dayjsObj: M {
'$L': 'en',
'$u': false,
'$d': 2020-12-30T07:00:00.000Z,
'$x': { '$timezone': 'America/New_York' },
'$y': 2020,
'$M': 11,
'$D': 30,
'$W': 3,
'$H': 7,
'$m': 0,
'$s': 0,
'$ms': 0,
'$offset': -300
},
'dayjsObj.format()': '2020-12-30T07:00:00-05:00',
offset: -300,
'dayjsObj.utcOffset(offset)': M {
'$L': 'en',
'$u': false,
'$d': 2020-12-30T12:00:00.000Z,
'$x': { '$localOffset': 300 },
'$y': 2020,
'$M': 11,
'$D': 30,
'$W': 3,
'$H': 12,
'$m': 0,
'$s': 0,
'$ms': 0,
'$offset': -300
},
'dayjsObj.utcOffset(offset).format()': '2020-12-30T12:00:00-05:00',
'dayjsObj.local().format()': '2020-12-30T12:00:00+00:00',
'dayjsObj.local()': M {
'$L': 'en',
'$u': false,
'$d': 2020-12-30T12:00:00.000Z,
'$x': {},
'$y': 2020,
'$M': 11,
'$D': 30,
'$W': 3,
'$H': 12,
'$m': 0,
'$s': 0,
'$ms': 0
}
}
```
On another computer where `dayjsObj.local().format()` is `2020-12-30T07:00:00-05:00`, `dayjsObj.utcOffset(offset).format()` gives `2020-12-30T07:00:00-05:00`:
```
{
dayjsObj: M {
'$L': 'en',
'$u': false,
'$d': 2020-12-30T12:00:00.000Z,
'$x': { '$timezone': 'America/New_York' },
'$y': 2020,
'$M': 11,
'$D': 30,
'$W': 3,
'$H': 7,
'$m': 0,
'$s': 0,
'$ms': 0,
'$offset': -300
},
'dayjsObj.format()': '2020-12-30T07:00:00-05:00',
offset: -300,
'dayjsObj.utcOffset(offset)': M {
'$L': 'en',
'$u': false,
'$d': 2020-12-30T12:00:00.000Z,
'$x': { '$localOffset': 300 },
'$y': 2020,
'$M': 11,
'$D': 30,
'$W': 3,
'$H': 7,
'$m': 0,
'$s': 0,
'$ms': 0,
'$offset': -300
},
'dayjsObj.utcOffset(offset).format()': '2020-12-30T07:00:00-05:00',
'dayjsObj.local().format()': '2020-12-30T07:00:00-05:00',
'dayjsObj.local()': M {
'$L': 'en',
'$u': false,
'$d': 2020-12-30T12:00:00.000Z,
'$x': {},
'$y': 2020,
'$M': 11,
'$D': 30,
'$W': 3,
'$H': 7,
'$m': 0,
'$s': 0,
'$ms': 0
}
}
```
Despite `dayjsObj` being set to the same timezone (`America/New_York`), `utcOffset` ignores it and uses the `local` time instead. I believe this is the result of the following line of code:

https://github.com/iamkun/dayjs/blob/a9d7d0398d22ebd4bfc3812ca0134a97606d54d9/src/plugin/utc/index.js#L99

Is the intended behavior to use the local time regardless of the timezone that was set, or is it something to be fixed to respect the converted timezone?

Start at src/plugin/utc/index.js around the linked line 99 and reproduce the supplied utc/tz example on both local-time environments. Determine the intended utcOffset behavior after tz('America/New_York'); done means the behavior is resolved and the reported inconsistency no longer occurs.

Written by the indexing model from the issue text.