[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