Ability to choose the location of u-m temp space while upgrading from ubuntu N to N+1

When trying to update from one version of Ubuntu to the next one, people may encounter the following error : "not enough space on /, please free XXXMb of space to continue". Sometimes people have other disks (eventually external hard drives) that they would like to use as temp space during the update but they just can't.

The idea is to add an option to the upgrade utility that would, in case on lack of space on /, ask the user if he wants to provide another location for downloading the stuff.

This feature is probably (very) easy to add, and may help lots of people having small hard drives.