Add fallback report loading in case websockets fail

This also resolves a race condition scenario. Even if the report gets
rendered via websockets before the controller response is rendered then
the fallback script loads the report again. It's not the most beautiful
but probably okay until we replace websockts altogether.

I'm leaving websockets in at the moment because it can render the report
much quicker than polling can.

app/controllers/admin/reports_controller.rb

@@ -58,12 +58,12 @@ module Admin
     end
 
     def render_in_background
-      blob = ReportBlob.create_for_upload_later!(report_filename)
+      @blob = ReportBlob.create_for_upload_later!(report_filename)
 
       ReportJob.perform_later(
         report_class:, user: spree_current_user, params:,
         format: report_format,
-        blob:,
+        blob: @blob,
         channel: ScopedChannel.for_id(params[:uuid]),
       )
     end