Preprocess a layout to work out if its in layout builder UI

yourtheme.theme.php

<?php
/**
 * Implements hook_preprocess_HOOK() for your_layout().
 */
function your_theme_preprocess_your_layout(&$vars) {
  // Note this only works for 'defaults'. You could adapt it for OverridesSectionStorage.
  // Ascertain if we're in layout-builder edit mode.
  $parameters = \Drupal::routeMatch()->getParameters();
  $vars['is_layout_builder'] = FALSE;
  $cache = new CacheableMetadata();
  $cache->addCacheContexts(['route', 'user.permissions']);
  $cache->applyTo($vars);
  if ($parameters->has('section_storage')) {
    try {
      $storage = $parameters->get('section_storage');
      if ($storage instanceof DefaultsSectionStorage &&
        ($display = $storage->getContextValue('display')) &&
        $display->getTargetEntityTypeId() === 'your_target_entity_type_id' &&
        $display->getTargetBundle() === 'your_target_bundle') {
        $vars['is_layout_builder'] = TRUE;
      }
    }
    catch (\Exception $e) {
      // Gulp.
    }
  }
}