Add ability to prevent cabal from deleting intermediates

I'm currently trying to debug `Setup` issue and have found that `cabal-install` makes this much harder than necessary as it deletes the linked `Setup` executable after running. It would be nice if `cabal-install` would grow a flag to preserve intermediates like this to ease debugging.

Start by tracing cabal-install's cleanup after the linked Setup executable runs, then inspect how command-line flags are handled. The requested behavior is a flag that preserves intermediates for debugging; done means the Setup executable remains when the flag is enabled, with coverage for that behavior.

Written by the indexing model from the issue text.