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

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.