mkdir -p LOG_DIR before writing the optional step log so that a race
with cleanup_build_log (EXIT trap archiving the log dir) does not cause
a "Directory nonexistent" error during lb binary_checksums / lb binary_iso.
Also downgrade apt-get update failure to a warning so a transient mirror
outage does not block kernel ABI auto-detection when the apt cache is warm.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Michael Chus
e84c69d360