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/build.sh

@@ -57,6 +57,7 @@ OVERLAY_STAGE_DIR="${DIST_DIR}/overlay-stage-${BUILD_VARIANT}"
 export BEE_GPU_VENDOR BEE_NVIDIA_MODULE_FLAVOR BUILD_VARIANT
 
 . "${BUILDER_DIR}/VERSIONS"
+export MEMTEST_VERSION
 export PATH="$PATH:/usr/local/go/bin"
 : "${BEE_REQUIRE_MEMTEST:=0}"