`--nimbleDir:someDir` doesn't work if someDir is relative
```
nimble install -y --nimbleDir:build/deps_tmp2 opengl
    Prompt: No local packages.json found, download it from internet? -> [forced yes]
Downloading official package list
    Success Package list downloaded.
Downloading https://github.com/nim-lang/opengl using git
/Users/timothee/git_clone/nim/Nim_devel/lib/pure/includes/oserr.nim(94) raiseOSError
Error: unhandled exception: No such file or directory
Additional info: "build/deps_tmp2" [OSError]
```

nimble v0.11.0 compiled at 2020-04-04 14:23:52
git hash: 4007b2a778429a978e12307bf13a038029b4c4d9

