trap RETURN is a bash extension not supported by /bin/sh on Debian.
With set -e active the unsupported trap call exited the build immediately
after lb build, before bootloader sync and ISO copy steps ran.
Remove both trap RETURN lines — explicit rm -rf at the end of the
function is sufficient for cleanup on the happy path.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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