Use MEPHI mirror, disable security repo, fix memtest in ISO build

- Switch all lb mirrors to mirror.mephi.ru/debian/ for faster/reliable downloads
- Disable security repo (--security false) — not needed for LiveCD
- Pin MEMTEST_VERSION=6.10-4 in VERSIONS, export to hook environment
- Set BEE_REQUIRE_MEMTEST=1 in build-in-container.sh — missing memtest is now fatal
- Fix 9100-memtest.hook.binary: add apt-get download fallback when lb
  binary_memtest has already purged the package cache; handle both 5.x
  (memtest86+x64.bin) and 6.x (memtest86+.bin) BIOS binary naming

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

iso/builder/auto/config

@@ -23,10 +23,10 @@ lb config noauto \
     --bootloaders "grub-efi,syslinux" \
     --debian-installer none \
     --archive-areas "main contrib non-free non-free-firmware" \
-    --mirror-bootstrap "https://deb.debian.org/debian" \
-    --mirror-chroot "https://deb.debian.org/debian" \
-    --mirror-binary "https://deb.debian.org/debian" \
-    --security true \
+    --mirror-bootstrap "http://mirror.mephi.ru/debian/" \
+    --mirror-chroot "http://mirror.mephi.ru/debian/" \
+    --mirror-binary "http://mirror.mephi.ru/debian/" \
+    --security false \
     --linux-flavours "amd64" \
     --linux-packages "${LB_LINUX_PACKAGES}" \
     --memtest memtest86+ \