Make the staged ISO copy writable on macOS

macOS's openrsync ignores --chmod, so the staged directories kept the
read-only permissions of the UDF mount and wimlib-imagex split failed
with Permission denied. Run chmod -R u+rwX on the staging directory on
Darwin.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

build-windows-install-disk.sh

@@ -214,6 +214,11 @@ step "Staging the ISO files without install.wim"
 # The UDF mount is read-only, so its directories are too; --chmod keeps the copies
 # writable, otherwise cleanup cannot remove them.
 run rsync -rt --chmod=u+rwX --exclude=/sources/install.wim "$mount_dir/" "$stage_dir/"
+if [[ "$os" == Darwin ]]; then
+  # macOS's openrsync ignores --chmod, so make the copies writable here (the split
+  # below writes into sources/).
+  run chmod -R u+rwX "$stage_dir"
+fi
 
 # Parts must stay below the 4 GiB FAT32 file limit; wimlib takes MiB.
 step "Splitting install.wim into install.swm parts (FAT32 files must stay below 4 GiB)"