cabal new-build doesn't produce logs for local packages

As far as I can tell `cabal new-build` no longer produces log files for local package builds. For instance,

```
$ git clone https://github.com/haskell-hvr/cabal-plan
$ cd cabal-plan
$ cabal new-build all
$ find -iname '*.log' | wc -l
0
```

Moreover, `-v2` shows no indication that a log was generated. I have a vague sense that this used to work so I suspect there is a regression here.