week() with timezone is incorrect. (Europe/London)

**Describe the bug**

Performance is inconsistent with momentjs.

see https://codesandbox.io/s/blue-platform-4zlug?file=/src/index.js

```js
console.log(
'dayjs: 2021-01-05',
dayjs.tz('2021-01-05', 'Europe/London').format("gggg-wo"),
dayjs.tz('2021-01-05', 'Europe/London').week()
);

// output
// dayjs: 2021-01-05 2021-6th 6

console.log(
'moment: 2021-01-05',
moment.tz('2021-01-05', 'Europe/London').format("gggg-wo"),
moment.tz('2021-01-05', 'Europe/London').week()
);

// output
// moment: 2021-01-05 2021-2th 2

```

**Expected behavior**

output `dayjs: 2021-01-05 2021-2th 2`

**Information**
- Day.js Version 1.10.7
- OS: MacOs
- Browser chrome 96.0.4664.93
- Time zone: GMT+0800

Start by running the linked CodeSandbox and compare dayjs.tz(..., 'Europe/London').week() and format("gggg-wo") with moment.tz for 2021-01-05. Trace the timezone and week entry points to determine why Day.js returns week 6 instead of 2. Done means the Europe/London example matches the expected Day.js output and the behavior is covered by a regression check.

Written by the indexing model from the issue text.