Fix grub-pc directory missing before writing grub.cfg

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/build.sh

@@ -955,6 +955,7 @@ elif [ -d "${LB_PKG_CACHE}" ] && [ "$(ls -A "${LB_PKG_CACHE}" 2>/dev/null)" ]; t
 fi
 
 if [ "$BEE_GPU_VENDOR" != "nvidia" ] || [ "$BEE_NVIDIA_MODULE_FLAVOR" != "proprietary" ]; then
+    mkdir -p "${BUILD_WORK_DIR}/config/bootloaders/grub-pc"
     cat > "${BUILD_WORK_DIR}/config/bootloaders/grub-pc/grub.cfg" <<'EOF'
 source /boot/grub/config.cfg