buildLocalCabal: use new cabal2nix feature

This changes `buildLocalCabal` so it doesn't rely on `sed` hacks anymore. I'm not sure if we should even add cabalInstall to buildInputs by default? (The old `buildLocalCabal` did it). IMO, `buildLocalCabal` should only provide the basic expression. Users can override that to add cabalInstall if they want to nix-shell in it themselves then, but I haven't changed that for now.