Running sketchtool inside plugin causes kernel panics
When trying to run the following code in a plugin on a sketchfile with 1 artboard and that one made exportable, my computer (latest macOS, MBP 13" 2017) freezes and then kernel panics.

```js
const absolutePath = sketch.fromNative(context.document).path;

execSync(`/Applications/Sketch.app/Contents/Resources/sketchtool/bin/sketchtool export slices ${absolutePath} --output /Users/thibault/ --format="png" --scales="1, 2, 3"`);
```

```console
$ npm start
bash: /usr/local/opt/node@8/bin/npm: /usr/bin/env: bad interpreter: Too many open files in system
bash: /Users/thibault/.pyenv/shims/powerline-shell: /usr/bin/env: badinterpreter: Too many open files in system
```