Use `user.query`, not `user.find`, in `arc tasks`

Summary: I'm going to deprecate `user.find`, `user.query` is more modern/powerful and obsoletes it.

Test Plan: Ran `arc tasks --owner epriestley`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8070