bootloaders: fix ignoring LB_DEBIAN_INSTALLER_GUI in menu creation

LB_DEBIAN_INSTALLER_GUI defines whether or not to provide the graphical installer. the installer_debian-installer script pays attention to it and does not download it if not wanted. the actual bootloaders however (both grub2/loopback and syslinux) ignore it, which leaves broken and unwanted menu entries. this fixes that.

Closes: #952890