Web "Update now" button fails to update

### Prerequisites

- [X] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) and found no answer

- [X] I have searched other issues and found no duplicates

- [X] I want to report a bug and not [ask a question or ask for help](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a)

- [X] I have set up AdGuard Home correctly and [configured clients to use it](https://github.com/AdguardTeam/AdGuardHome/wiki/Clients). (Use the [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) for help with installing and configuring clients.)

### Platform (OS and CPU architecture)

Linux/MIPS64, on EdgeRouter ER-12, mips64_softfloat arch

### Installation

GitHub releases or script from README

### Setup

On a router, DHCP is handled by the router

### AdGuard Home version

v0.107.29

### Action

Clicked the "Update now" button in the web UI.

### Expected result

Update works

### Actual result

Update fails. Error in the logs:

```
Jun 17 20:12:47 router AdGuardHome[4966]: 2023/06/17 20:12:47.327120 [info] updater: updating
Jun 17 20:12:53 router AdGuardHome[4966]: 2023/06/17 20:12:53.331274 [error] updater: failed: replacing: rename /config/AdGuardHome/AdGuardHome /tmp/AdGuardHome/agh-backup/AdGuardHome: invalid cross-device link
```

Note that running from the CLI as root `./AdGuardHome --update` succeeds, it's the Web UI that fails. Running the CLI from where AdGuardHome is installed works because it doesn't use `/tmp` (which is tmpfs) but it does leave a mess behind, a backup dir `agh-backup/` and also other files it extracts, README.md, etc. I think the updater should be nicer and clean up after itself or at least put everything in a directory that I can easily delete and not have to clean up a bunch of files by hand.

### Additional information and/or screenshots

EdgeRouter ER-12, EdgeOS