Hand off white-label token before navigating into the standalone builder page

WorkflowStudio's Duplicate/create/open-builder actions router.push into
/workflow/[id]/builder, a top-level route with no white-label session of its
own. Stash the apiKey in sessionStorage under wl_workflow_token right before
navigating so that page can pick it up and thread it into the builder's own
axios instance (see workflow-ui's matching fix).