build(iso): switch default Debian mirror, add --mirror override

mirror.mephi.ru was timing out during lb binary_rootfs. Default to
deb.debian.org (Fastly-backed, geo-balanced) and allow overriding via
--mirror on build-in-container.sh/build.sh, or BEE_MIRROR in auto/config.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

iso/builder/auto/config

@@ -23,6 +23,7 @@ else
 fi
 
 LB_HOSTNAME="${BEE_HOSTNAME:-easy-bee}"
+LB_MIRROR="${BEE_MIRROR:-http://deb.debian.org/debian/}"
 
 lb config noauto \
     --distribution bookworm \
@@ -31,9 +32,9 @@ lb config noauto \
     --bootloaders "grub-efi,syslinux" \
     --debian-installer none \
     --archive-areas "main contrib non-free non-free-firmware" \
-    --mirror-bootstrap "http://mirror.mephi.ru/debian/" \
-    --mirror-chroot "http://mirror.mephi.ru/debian/" \
-    --mirror-binary "http://mirror.mephi.ru/debian/" \
+    --mirror-bootstrap "${LB_MIRROR}" \
+    --mirror-chroot "${LB_MIRROR}" \
+    --mirror-binary "${LB_MIRROR}" \
     --security true \
     --linux-flavours "amd64" \
     --linux-packages "${LB_LINUX_PACKAGES}" \