Fix assigned user dropdown on task create [SCI-12207]

app/controllers/experiments_controller.rb

@@ -47,7 +47,7 @@ class ExperimentsController < ApplicationController
   end
 
   def assigned_users
-    render json: User.where(id: @experiment.user_assignments.select(:user_id)),
+    render json: @experiment.users,
            each_serializer: UserSerializer,
            user: current_user
   end