Make --conduit-token work without requiring .arcrc

Summary:
Ref T5955. This logic is a little cleaner than the previous version.

Don't require ~/.arcrc to exist if the caller provides --conduit-token.

Test Plan:

• Made calls with --conduit-token and no ~/.arcrc.
• Made calls with --conduit-token and a normal ~/.arcrc.
• Made calls with normal arc.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: mbishopim3, epriestley

Maniphest Tasks: T5955

Differential Revision: https://secure.phabricator.com/D12750