Use haskell-process-path-cabal everywhere

When running `haskell-process-cabal-build`, I was surprised by errors like:

```
-> ":!cd /home/bergey/code/diagrams/lib/ && cabal build
"
<- "/bin/sh: cabal: command not found
```

and further surprised that setting `haskell-process-path-cabal` to an absolute path didn't fix the errors.  With the patch below, I can build my projects, but maybe there's a better way?