Specifying --with-ghc and local dependencies does not work well

This is probably a very niche concern but if I specify --with-ghc with a relative path but also have local dependencies then the build will fail.

For example,

```
cabal install --with-ghc=inplace/bin/ghc-stage2 numtype-dk/ dimensional-1.0.1.1
```

Then cabal will cd into `numtype-dk` directory and no longer be able to find the compiler. If i give the full path then everything works as expected.
