`--distdir` and `--ghcjs` don’t go well together

In https://travis-ci.org/nomeata/incredible/builds/407350923 I stumbled over this:
```
cd logic && cabal new-build --distdir=dist-newstyle-ghcjs --ghcjs --dependencies-only --disable-tests
cabal: '/opt/ghcjs/8.2/bin/ghcjs' exited with an error:
ghcjs: can't find a package database at
/home/travis/build/nomeata/incredible/logic/dist-newstyle/packagedb/ghc-8.2.2
```
Why is it not adhering to the `--distdir` that I gave?

This is using `cabal-install-head_2.3+git20180723.1.67bdbab-6~14.04_amd64.deb` … and weirdly it works locally (with a self-built `ghcjs` though)

@hvr any ideas?