Prevent reference_implementation to be copied to Runner

app/models/submission.rb

@@ -205,6 +205,7 @@ class Submission < ApplicationRecord
     begin
       runner = Runner.for(user, exercise.execution_environment)
       files = collect_files
+      files.reject!(&:reference_implementation?)
       files.reject!(&:teacher_defined_assessment?) if cause == 'run'
       Rails.logger.debug { "#{Time.zone.now.getutc.inspect}: Copying files to Runner #{runner.id} for #{user_type} #{user_id} and Submission #{id}." }
       runner.copy_files(files)