isoWeek() ignores artificially set timezone

**Describe the bug**
isoWeek() returns the correct result for the local timezone but not when using an arbitrary dayjs object with a timezone set.


On the first call, I had my timezone manually set to glascow/utc. One the second call, it's a local timezone of Paris.
`36` is correct, `35` is not. My understanding is that since a `tz` is set on the dayjs instance, the local timezone should not matter?

**Expected behavior**
It should always return the isoWeek for the set timezone.

**Information**
- dayjs `1.10.7`
- Latest chrome

Reproduce the issue described in #1655 with a Day.js object whose timezone is set, comparing isoWeek() in UTC/Glasgow and Paris. Trace the isoWeek and timezone entry points to find where the local timezone affects the result. Done when isoWeek() uses the instance's configured timezone, with a regression test covering the differing zones.

Written by the indexing model from the issue text.