docs: def check_for_executable(supernova_args, env_vars):
      """
      It's possible that a user might set their custom executable via an
      environment variable.  If we detect one, we should add it to supernova's
      arguments ONLY IF an executable wasn't set on the command line.  The
      command line executable must take priority.
      """
      exe = supernova_args.get('executable', 'default')
      if exe != 'default':
          return supernova_args
      if 'OS_EXECUTABLE' in env_vars.keys():
          supernova_args['executable'] = env_vars['OS_EXECUTABLE']
          return supernova_args
      supernova_args['executable'] = 'nova'
      return supernova_args