Cabal doesn't work when $HOME is not writable

If you ever try to write a Nix package that runs `cabal build`, you run into [this issue](https://github.com/NixOS/nixpkgs/issues/16144):

Config file /homeless-shelter/.cabal/config not found.
Writing default configuration to /homeless-shelter/.cabal/config
/homeless-shelter: createDirectory: permission denied (Permission denied)

This is because in the context of a Nix build, you don't have a home directory (and, I believe, Nix sets the `HOME` variable to `/homeless-shelter`, which is not a real location, just to be cute).

This isn't a *huge* problem - you can work around it by running `HOME=$TMP cabal build` instead to use the temporary directory as your home directory - but this seems like an unnecessary complication and I'm wondering if we could avoid it altogether.

Does Cabal actually *need* to write to `$HOME/.cabal`? Could it skip whatever it's trying to do there and just build the package?