buildFHSEnv: use LOCALE_ARCHIVE from environment if present

If the environment provides the variable (as NixOS does when the i18n option is
set), keep it rather than overriding unconditionally.

Fixes #354887

Author: Atemu

pkgs/build-support/build-fhsenv-bubblewrap/buildFHSEnv.nix

@@ -87,7 +87,7 @@ let
     destination = "/etc/profile";
     text = ''
       export PS1='${name}-fhsenv:\u@\h:\w\$ '
-      export LOCALE_ARCHIVE='/usr/lib/locale/locale-archive'
+      export LOCALE_ARCHIVE="''${LOCALE_ARCHIVE:-/usr/lib/locale/locale-archive}"
       export PATH="/run/wrappers/bin:/usr/bin:/usr/sbin:$PATH"
       export TZDIR='/etc/zoneinfo'