[nix-local-build] Replacement needed for `cabal copy`

When preparing binary distributions of Cabalized software, I use a combination of `configure` and `copy` to prepare the staging directory that will be turned into a tarball, `.msi`, etc. For example:

``` zsh
% cabal configure --prefix=/usr/local
% cabal build
% cabal copy --destdir=$(pwd)/foo-1.0
% tar -czf foo-1.0.tar.gz foo-1.0 # for example
```

It looks like we can still specify a `--prefix` argument, but there is no new version of `copy`. I imagine a solution for this might overlap with #3332 but I want to make a separate ticket since the use cases are distinct.