Fix an exception with user cache generation in "bin/conduit call --as <user>"

Summary:
Ref T13249. Using "--as" to call some Conduit methods as a user can currently fatal when trying to access settings/preferences.

Allow inline regeneration of user caches.

Test Plan: Called project.edit to add a member. Before: constructing a policy field tried to access the user's preferences and failed. After: Smooth sailing.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13249

Differential Revision: https://secure.phabricator.com/D20255