Log Message:
Update drupal8 to 8.6.4

Many bug fixes including:
- Breadcrumbs disappears when starting with front-page after cache rebuild
- Adding a display mode to a content type using layout, and disabling
  layout on that new display mode removes the layout_builder__layout
  field and breaks layout in already configured display modes
- Clearing the persistent entity cache every time we switch between
  workspaces is super wasteful

For full list, see:
- https://www.drupal.org/project/drupal/releases/8.6.3
- https://www.drupal.org/project/drupal/releases/8.6.4