perf: cache apk packages between ISO builds

Keep apks_* workdir sections so packages aren't re-downloaded on each build.
Only non-apks sections (kernel, apkovl, final image) are cleaned to pick up changes.

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

iso/builder/build-debug.sh

@@ -98,8 +98,12 @@ cp "${BUILDER_DIR}/genapkovl-bee_debug.sh" "${HOME}/.mkimage/"
 # Export overlay dir so the profile script can find it regardless of SRCDIR.
 export BEE_OVERLAY_DIR="${OVERLAY_DIR}"
 
-# Clean workdir so apkovl changes are always picked up (kernel/apks sections are re-cached).
-rm -rf /var/tmp/bee-iso-work
+# Clean workdir selectively: remove everything except apks cache so packages aren't re-downloaded.
+# mkimage stores each section in a hash-named subdir; apks_* dirs contain downloaded packages.
+if [ -d /var/tmp/bee-iso-work ]; then
+    find /var/tmp/bee-iso-work -maxdepth 1 -mindepth 1 \
+        -not -name 'apks_*' -exec rm -rf {} + 2>/dev/null || true
+fi
 
 # Run from /var/tmp to avoid git repo context conflicts and to ensure enough scratch space.
 # mkinitfs/update-kernel use TMPDIR for initramfs build; tmpfs /tmp is only ~1GB.