Flush all caches once done

Discussed in Slack and I am convinced that, although the more aggressive cache flush is a ding for performance, that concern is outweighed by the utterly baffling shittiness of a cache that has been accidentally primed with garbage during the install process, rendering Drupal non-functional unless you are able to drush cr.

Automatically closed - issue fixed for 2 weeks with no activity.

Maddeningly, I had to revert this because it turns out that flushing all caches wipes the session, forcing you to log in after the installation is complete. It's not clear why this happens, and it also seems to be semi-random (affecting 11.1.x but...not 11.2.x? maybe? it's not clear).

drupal_flush_all_caches() is a very aggressive cache wipe so this probably makes sense to revert for now and postpone until there is a CI pipeline with test coverage.