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/build.sh

@@ -24,6 +24,7 @@ while [ $# -gt 0 ]; do
     case "$1" in
         --authorized-keys) AUTH_KEYS="$2"; shift 2 ;;
         --variant) BUILD_VARIANT="$2"; shift 2 ;;
+        --mirror) BEE_MIRROR="$2"; shift 2 ;;
         *) echo "unknown arg: $1"; exit 1 ;;
     esac
 done
@@ -55,6 +56,9 @@ BUILD_WORK_DIR="${DIST_DIR}/cache/live-build-work-${BUILD_VARIANT}"
 OVERLAY_STAGE_DIR="${DIST_DIR}/cache/overlay-stage-${BUILD_VARIANT}"
 
 export BEE_GPU_VENDOR BEE_NVIDIA_MODULE_FLAVOR BUILD_VARIANT
+if [ -n "${BEE_MIRROR:-}" ]; then
+    export BEE_MIRROR
+fi
 
 . "${BUILDER_DIR}/VERSIONS"
 export MEMTEST_VERSION