When building the ISO we use grub-mkrescue to setup the outer GRUB on
the ISO that's used to boot the actual installer, but mkrescue sadly
has no native support to copy over the signed shim, so add that but
only enable it through an environment variable so that we do not have
to vet this overly closely as it won't affect any normal grub use
anyway, even less so as mkrescue is used rather rarely on running
systems.
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
