--ipid doesn't work with cabal install?

I am trying to use the `--ipid` flag to install a package so it ends up with `pkg-0.1.conf` rather than `pkg-0.1-asdf.conf`. It seems the `--ipid` flag should do this.

I invoke GHC like this:

```
/nix/store/iq76wsmhvmidnyjkbgiklfi4zpwxld7h-ghc-build-environment/bin/cabal install groups/ --with-compiler /nix/store/iq76wsmhvmidnyjkbgiklfi4zpwxld7h-ghc-build-environment/bin/ghc --package-db _build/stage0/lib/package.conf.d --ipid=$pkg-$version
```

but the package is still registered with the suffix. Is this broken or am I doing something wrong?