Chromium-based apps can't read timezone in certain timezone

Step to reproduce:
1. Setup an Ubuntu Core 22 VM. Then, set timezone to "Asia/Bangkok": `sudo timedatectl set-timezone Asia/Bangkok`.
2. Install `ubuntu-frame`. Then, build and install the Electron version of `iot-example-graphical-snap` [1].
3. Start the snap. Go to "View > Toggle Developer Tools". Then, go to "Console" tab and type: `new Date()`.
- Expected result: the time is 7 hours ahead of UTC, and ends with "GMT+0700 (Indochina Time)".
- Actual result: the time is in UTC, and ends with "GMT+0000 (GMT)".

I have a full analysis, done under Ubuntu Touch which also use writable-paths system, at [2]. But basically, ICU tries to determine the timezone by reading symlink target of `/etc/localtime`, which then fails. This isn't failure on its own (it can still understand `+07` from GLibC), but then Chromium tries to pass `+07` to Zygote process, which ICU can't deserialize back to a timezone falling back to `Etc/Unknown`.

We can either patch ICU so that it understand `/etc/writable` redirection (similar to what we've done with systemd), or ~~patch Chromium to correct the passed zone ID (to something like `GMT+07`)~~ turns out you can't do that because of time zone abbreviation collision (e.g. PST = Pacific Standard Time = Philippines Standard Time). I feel like patching ICU could have a wider effect and would make stuff more functional.

To see the effect of the ICU problem alone, installs `wpe-webkit-mir-kiosk` instead and run `snap set wpe-webkit-mir-kiosk url="https://www.w3schools.com/js/tryit.asp?filename=tryjs_date_current"`.

[1] https://github.com/MirServer/iot-example-graphical-snap/tree/22/Electron-quick-start
[2] https://gitlab.com/ubports/development/core/packaging/qtwebengine/-/issues/15