Pass changed paths to unit tests process

Summary:
Sets an environment variable which will then be read from within the node process running unit tests in rNEXT.

This does not use `$future->updateEnv` as I found this function reset the existing environment on my machine.

Reviewers: tail, matt, branden, BYK

Reviewed By: BYK

Subscribers: gabalafou

Maniphest Tasks: T11100

Differential Revision: http://phabricator.local.disqus.net/D14985