Offer stable version of Arcanist

Summary:
Perhaps useful to provide a stable version of the tool
during the workflow migration.

Test Plan:

$ arc-stable # like arc, but run nasqueron/arcanist:stable
$ arc which

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: Nasqueron Docker deployment squad

Tags: Docker images

Differential Revision: https://devcentral.nasqueron.org/D2242