Use lb clean --all to clear bootstrap cache on every build

Prevents stale debootstrap cache from bypassing --debootstrap-options
changes (e.g. --include=ca-certificates added in v8.15).

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

iso/builder/build.sh

@@ -1305,7 +1305,7 @@ BEE_GPU_VENDOR_UPPER="$(echo "${BUILD_VARIANT}" | tr 'a-z-' 'A-Z_')"
 export BEE_GPU_VENDOR_UPPER
 
 cd "${LB_DIR}"
-run_step_sh "live-build clean" "80-lb-clean" "lb clean 2>&1 | tail -3"
+run_step_sh "live-build clean" "80-lb-clean" "lb clean --all 2>&1 | tail -3"
 run_step_sh "live-build config" "81-lb-config" "lb config 2>&1 | tail -5"
 dump_memtest_debug "pre-build" "${LB_DIR}"
 run_step_sh "live-build build" "90-lb-build" "lb build 2>&1"