Drop the redundant mkdir of the staged sources directory

rsync already creates sources/ because the ISO has one.

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

build-windows-install-disk.sh

@@ -212,7 +212,6 @@ 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/"
-run mkdir -p "$stage_dir/sources"
 
 # 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)"