`stack --work-dir` fails with directories above the current directory

#### Steps to reproduce:
1. In any stack project with a parent directory, run `stack build --work-dir=../.stack-work`
#### Expected:

The build proceeds using the `.stack-work` directory of the parent directory
#### Actual:

Stack errors out with

```
InvalidRelDir "../../.stack-work"
```

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Seems like this ought to work, right?