Allow to run arc commands as root

Summary:
By allowing to run arc commands as user, the /root as home use case
has been forgotten. Root should use standard image and legacy wirings.

This is a follow-up for 4d419de04afb.

Test Plan: arc shell and sudo arc shell with and without image built on Dwellers.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2343