artifact-reference should support non-public artifacts

[escapewindow]

mozilla-extensions/xpi-template#37
Essentially, we probably want to remove this check.

Given that:

a) we can use artifact-reference for scriptworkers, which often do have private artifact scopes baked in (until we resolve mozilla-releng/scriptworker#426, after which we'll need to grant the scope to the task as normal), and
b) if we grant the given task the proper private artifact scope and enable the taskcluster-proxy, non-scriptworker tasks can also download private artifacts,

I don't see a need for this artificial restriction.

[jcristau]

Fixed by #485