set custom arcrc file earlier

Summary: It appears to have never really work? At least as far as phabricator.uri (empirically).

Test Plan:
removed ~/.arcrc, run call-conduit user.whoami with --arcrc-file and --trace.
set-config, get-config and alias also read and write to the right place now.

Reviewers: avivey

Reviewed By: avivey

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9263