Don't try to run our fancy stuff for 'arc install-certificate'.

Summary:
Our fancy --cc-completion and the like requires using the arc
certificate to query phabricator.  That won't work before the arc
certificate is installed!  'arc install-certificate' doesn't need to
use any of our fancy add-ons anyway, so it's fine just skipping them.

Test Plan:
Ran
   arc install-certificate
and
   arc help
and both successfully did their thang.

Reviewers: alpert, benkomalo

Reviewed By: benkomalo

Subscribers: brian, colinfuller

Differential Revision: http://phabricator.khanacademy.org/D8400