Error if `--enable-nix` is passed to `v2-command`

It probably (hopefully) does nothing, but seems to confuse at least one user.

Alternatively, make `v2-build` etc do what it says, i.e.

```
--enable-nix Enable Nix integration: run commands through
nix-shell if a 'shell.nix' file exists
```

EDIT: I think it could possibly work, if `cabal-hash.txt` picks everything relevant. Probably cache won't be hit that often though. Also one would require that `~/.cabal/store` places indirect nix gc-roots. In short, I think it's hard to make `--enable-nix` work well, instead I'd recommend users to run `nix-shell` themselves and run `cabal v2-*` commands inside.