as requested in #11098 this add the cache cleaning for the backend cache see #11098 for the error situation and for testing instructions. This might can be merged on review.
I enabled cache and made sure it was working for the admin (make sure it shows more than _system and com_plugins
I applied the patch and then reinstalled Joomla using the new ability to upload a package in com_joomlaupdate
I checked the cache and it had been cleaned (note it will never show empty because as soon as you open com-cache it will show _system and com_plugins)
Awesome - thanks @rdeutz