[Error] HOST_LOCALE not obtained correctly when /etc/locale.conf doesn't exist

**Describe the bug**
Currently, when creating a distrobox on a host system where /etc/locale.conf does not exist, the container's LANG variable will be set to a locale that's not available in the container.

For example, in gentoo, the locale is set in [/etc/env.d/02locale](https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Base#Locale_selection) instead of /etc/locale.conf, which the distrobox-init script never reads, and instead initializes the HOST_LOCALE variable to a fallback value.

I'd suggest having distrobox pick up the LANG variable from the host environment, instead of trying to parse host system configuration. Alternatively, distrobox should provide a way to set HOST_LOCALE in the configuration, and set the container's LANG variable to the same value.

This is a follow-up of https://github.com/89luca89/distrobox/issues/1743

查看設定 HOST_LOCALE 的 distrobox-init 指令碼，可能位於 /usr/local/bin/distrobox-init 或類似位置。該指令碼目前讀取 /etc/locale.conf；修改它，使其同時檢查主機的 LANG 環境變數，或在 Gentoo 上讀取 /etc/env.d/02locale。透過在 Gentoo 主機或沒有 /etc/locale.conf 的系統上建立容器進行測試，並驗證容器的 LANG 與主機的 locale 相符。

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