Cabal sometimes doesn't delete empty temporary directories

Can be reproducing by running the bootstrap script. After it finished, there will be several empty `cabal-$NUM` directories in `/tmp`.

Start by reproducing the issue with the bootstrap script and inspect the generated cabal-$NUM directories in /tmp after it finishes. Trace where those temporary directories are created and cleaned up; done means the bootstrap run leaves no empty cabal-$NUM directories behind.

Written by the indexing model from the issue text.