Bug#1004274: debian-installer: graphical installer breaks debconf in rescue shell

[Pascal Hambourg <pascal@plouf.fr.eu.org>]

Package: debian-installer
Version: 20210731+deb11u2
Tags: d-i

Hello,

Steps to reproduce:
- Boot the Debian installer
- At the boot menu, select "Graphical rescue mode"
- follow steps until "Enter rescue mode"
- Select a Debian root device
- Select "Execute a shell in <selected root device>"
- in the shell, execute "dpkg-reconfigure keyboard-configuration"
(any other installed package than keyboard-configuration which has 
configuration questions will do too)

Output:
debconf: unable to initialize frontend: Newt
debconf: (Can't locate Debconf/FrontEnd/Newt.pm in @INC...)
debconf: falling to frontend: Noninteractive

Forcing --frontend=dialog or readline does not help.

This seems to be caused by the DEBIAN_FRONTEND=newt environment variable 
leaked from the graphical installer. It does not happen in the text 
installer nor in tty2/tty3 shells.

Not sure if this bug should be assigned to debian-installer, rescue-mode 
or debconf which should ignore DEBIAN_FRONTEND if invalid or superseded 
with --frontend. The issue also happens when selecting "Execute a shell" 
from the main menu and executing chroot manually, so it is not specific 
to rescue-mode.