Set the working directory to the project root for external linter `ExecFuture` classes.

Summary: Fixes T5085. Currently, the `ExecFuture` instances that are used to call an external linter are executed in the current working directory. This means that if a path is specified in the `.arclint` file, relative to the project root directory, that the path will not be properly interpreted by the external linter when `arc lint` is called from a level deeper than the project root.

Test Plan: Ran `arc lint` from a subdirectory of a project and verified that the linter did not throw an exception due to not being able to find the specified configuration file.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5085

Differential Revision: https://secure.phabricator.com/D9159