$u value is removed when modifying a time zone-aware dayjs object

**Describe the bug**
Dayjs objects created by modifying an existing time zone-aware object using `.hour()` (or any other setter) do not have the `$u` field. This causes Jest's `toEqual` assertion to fail even though both objects point to the same moment in time and have the same time zone offset.

```typescript
import dayjs from "dayjs";
import timezone from "dayjs/plugin/timezone";
import utc from "dayjs/plugin/utc";
import customParseFormat from "dayjs/plugin/customParseFormat";

dayjs.extend(utc);
dayjs.extend(timezone);
// Uncomment to make the test pass
// dayjs.extend(customParseFormat);

describe("Dayjs", () => {
it("$u information is lost after setting time", () => {
const someDate = dayjs().tz('Europe/London');
// Use the setter to create a new dayjs object with the same hour
// Before running the test set the value to the current time in London
const someDateWithHourSpecifiedUsingSetter = someDate.hour(20);

expect(someDate).toEqual(someDateWithHourSpecifiedUsingSetter);
});
});
```

Result:

**Expected behavior**
The test should pass no matter if `dayjs` was extended with `customParseFormat`.

**Information**
- Day.js Version: v1.11.3
- OS: macOS
- ~Browser:~ Node: v16
- Time zone: Europe/Berlin (UTC+2)

Start with the timezone and utc plugins and trace how the `.hour()` setter creates a new object from `dayjs().tz('Europe/London')`. Reproduce the Jest `toEqual` case with and without `customParseFormat`; done means setter-created objects retain the `$u` information and the assertion passes in both configurations.

Written by the indexing model from the issue text.