Bug#586434: Let's clean the apt cache

[Frans Pop <elendil@planet.nl>]

On Sunday 20 June 2010, Frans Pop wrote:
> The main reason IIRC is that leaving the packages makes it unnecessary
> to download them again if part of e.g. tasksel fails for whatever reason
> and the user has to install some packages manually [...].

Note that the above argument is only really valid for packages installed 
*after* base-installer. If base-installer fails we generally consider the 
whole installation broken so only cleaning the cache at the end of that 
(and not also during finish-install) could be something to consider.