Add i386 custom installer for amd64 builds

This is a follow-up of commit e94dd330.

debian-cd wants both the amd64 and i386 installer images for amd64 builds, so in case we're using a custom installer, we need to copy both.

Otherwise, build fails:

DEBUG build/debian-cd stdout:   Adding Release files
DEBUG build/debian-cd stdout:   Trying to add upgrade* directories
DEBUG build/debian-cd stdout:   (Optionally) making the image bootable for amd64:
DEBUG build/debian-cd stdout:     Running tools/boot/kali-last-snapshot/boot-amd64 1 /scratch/live-build-config/simple-cdd/tmp/cd-build/kali-last-snapshot/CD1
DEBUG build/debian-cd stderr: E: No packages found
DEBUG build/debian-cd stderr: cp: cannot stat '<<WORKDIR>>/simple-cdd/tmp/mirror/dists/kali-last-snapshot/main/installer-i386/current/images/cdrom/debian-cd_info.tar.gz': No such file or directory
DEBUG build/debian-cd stdout:       FAILED: error 1
DEBUG build/debian-cd stderr:     Failed to start disc 1, error 256
DEBUG build/debian-cd stderr: make: *** [Makefile:487: image-trees] Error 1