Fix optional step log dir missing after memtest recovery

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>

iso/builder/build.sh

@@ -775,6 +775,7 @@ run_optional_step_sh() {
         return 0
     fi
 
+    mkdir -p "${LOG_DIR}" 2>/dev/null || true
     step_log="${LOG_DIR}/${step_slug}.log"
     echo ""
     echo "=== optional step: ${step_name} ==="
@@ -798,13 +799,14 @@ start_build_log
 # install them on the fly so NVIDIA modules and ISO kernel always match.
 if [ -z "${DEBIAN_KERNEL_ABI}" ] || [ "${DEBIAN_KERNEL_ABI}" = "auto" ]; then
     echo "=== refreshing apt index to detect current kernel ABI ==="
-    apt-get update -qq
+    apt-get update -qq || echo "WARNING: apt-get update failed, trying cached index"
     DEBIAN_KERNEL_ABI=$(apt-cache depends linux-image-amd64 2>/dev/null \
         | awk '/Depends:.*linux-image-[0-9]/{print $2}' \
         | grep -oE '[0-9]+\.[0-9]+\.[0-9]+-[0-9]+' \
         | head -1)
     if [ -z "${DEBIAN_KERNEL_ABI}" ]; then
         echo "ERROR: could not auto-detect kernel ABI from apt-cache" >&2
+        echo "Hint: set DEBIAN_KERNEL_ABI=x.y.z-N in iso/builder/VERSIONS to skip auto-detection" >&2
         exit 1
     fi
     echo "=== kernel ABI: ${DEBIAN_KERNEL_ABI} ==="