Re-enable security repo: kernel 6.1.0-44 is in bookworm-security only

Disabling --security broke the build because linux-image-6.1.0-44-amd64
is a security update not present in the base bookworm repo.
Main packages already come from mirror.mephi.ru.

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

iso/builder/auto/config

@@ -26,7 +26,7 @@ lb config noauto \
     --mirror-bootstrap "http://mirror.mephi.ru/debian/" \
     --mirror-chroot "http://mirror.mephi.ru/debian/" \
     --mirror-binary "http://mirror.mephi.ru/debian/" \
-    --security false \
+    --security true \
     --linux-flavours "amd64" \
     --linux-packages "${LB_LINUX_PACKAGES}" \
     --memtest memtest86+ \