revert(iso): remove apt-cacher-ng support, use lb package cache instead

apt-cacher-ng requires a separate container; lb's own package cache
persisted in --cache-dir is simpler and sufficient.

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

iso/builder/auto/config

@@ -16,14 +16,6 @@ else
     LB_LINUX_PACKAGES="linux-image"
 fi
 
-# Route apt through proxy if BEE_APT_PROXY is set (e.g. apt-cacher-ng).
-# Usage: APT_PROXY=http://host:3142 ./build-in-container.sh
-if [ -n "${BEE_APT_PROXY:-}" ]; then
-    _MIRROR="${BEE_APT_PROXY%/}/debian"
-else
-    _MIRROR="https://deb.debian.org/debian"
-fi
-
 lb config noauto \
     --distribution bookworm \
     --architectures amd64 \
@@ -31,9 +23,9 @@ lb config noauto \
     --bootloaders "grub-efi,syslinux" \
     --debian-installer none \
     --archive-areas "main contrib non-free non-free-firmware" \
-    --mirror-bootstrap "${_MIRROR}" \
-    --mirror-chroot "${_MIRROR}" \
-    --mirror-binary "${_MIRROR}" \
+    --mirror-bootstrap "https://deb.debian.org/debian" \
+    --mirror-chroot "https://deb.debian.org/debian" \
+    --mirror-binary "https://deb.debian.org/debian" \
     --security true \
     --linux-flavours "amd64" \
     --linux-packages "${LB_LINUX_PACKAGES}" \