workingDir is not respected on multi-app configs

[giratikanon]

Setting workingDir in a multi-app config doesn't work, because spawnWith only looks for the main thread's workingDir: cli.js#L280. Each config's workingDir is merged into options, but spawnWith never gets changed.

So if you're in /home and run forever apps.json, and app.json looks like this:

[
  {
    ...
    "workingDir": "/app-1" 
  }, 
 {
    ...
    "workingDir": "/app-2" 
  }
]

But process.cwd() for app-1 and app-2 returns this:

/home

[indexzero]

This was fixed in 0.15.2