fix(memtest): extract EFI binary from .deb cache if chroot/boot/ is empty

memtest86+ postinst does not place files in /boot in a live-build chroot
without grub triggers. Added fallback: extract directly from the cached
.deb via dpkg-deb -x, with verbose logging throughout.

Also remove "NVIDIA no MSI-X" from boot menu (premature — root cause unknown).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/config/bootloaders/grub-pc/grub.cfg

@@ -24,11 +24,6 @@ menuentry "EASY-BEE (NVIDIA GSP=off)" {
     initrd  @INITRD_LIVE@
 }
 
-menuentry "EASY-BEE (NVIDIA no MSI-X)" {
-    linux   @KERNEL_LIVE@ @APPEND_LIVE@ nomodeset bee.nvidia.mode=nomsi net.ifnames=0 biosdevname=0 mitigations=off transparent_hugepage=always numa_balancing=disable nowatchdog nosoftlockup
-    initrd  @INITRD_LIVE@
-}
-
 menuentry "EASY-BEE (fail-safe)" {
     linux   @KERNEL_LIVE@ @APPEND_LIVE@ bee.nvidia.mode=gsp-off memtest noapic noapm nodma nomce nolapic nosmp vga=normal net.ifnames=0 biosdevname=0
     initrd  @INITRD_LIVE@