Using dayjs.tz.setDefault with endOf('day') makes the date tomorrow

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**Describe the bug**

If I set the global timezone with `dayjs.tz.setDefault()` and use `dayjs.tz()` with `endOf('day')` I get the next day.

- 2020-01-**08**T23:05:05-08:00 (date I used)
- 2020-01-**09**T23:59:59-08:00 (date I get)
- 2020-01-**08**T23:59:59-08:00 (date I should get)

[Example Code](https://codesandbox.io/s/dayjstzsetdefault-bug-f5ju6?file=/src/index.ts)

```ts
dayjs.extend(utc);
dayjs.extend(timezone);

dayjs.tz.setDefault("America/Los_Angeles");

dayjs.tz("2020-01-08T23:05:05-08:00").endOf("day").format(); // 2020-01-09T23:59:59-08:00 (wrong date)
```

Using `dayjs().tz().endOf("day")` works correctly but not `dayjs.tz().endOf("day")`:

```ts
// This works correctly:
dayjs("2020-01-08T23:05:05-08:00").tz("America/Los_Angeles").endOf("day").format(); // 2020-01-08T23:59:59-08:00

// This does not work correctly (same code above in the first example)
dayjs.tz.setDefault("America/Los_Angeles");
dayjs.tz("2020-01-08T23:05:05-08:00").endOf("day").format(); // 2020-01-09T23:59:59-08:00 (wrong date)
```

**Expected behavior**

I expect the end of day be the same of the date being passed in.

Also `startOf("day")` causes the same issue so I don't think it's only related to `endOf("day")` and `startOf("day")`.

**Information**
- Day.js Version 1.10.3
- OS: macOS Catalina 10.15.7
- Browser Google Chrome Version 87.0.4280.88
- Time zone: GMT-06:00 DST (Central Daylight Time)

Reproduce the discrepancy using the linked CodeSandbox and the dayjs.tz(), startOf('day'), and endOf('day') entry points. Compare the default-timezone path with dayjs().tz() and verify that both start and end of day preserve the input date in the expected timezone.

Written by the indexing model from the issue text.