--upgrade-cabal wants write access to system ghc installation

I tried 

```
stack setup --upgrade-cabal
```

in a project directory (that has `stack.yaml` with (default) `system-ghc: true`). It failed with

```
/opt/ghc/stable/new-cabal: createDirectory: permission denied (Permission denied)
```

That was unexpected. I think it should not want to do that.

```
$ which ghc
/opt/ghc/stable/bin/ghc

$ stack --version
Version 1.0.4.3 x86_64
```
