old_contents: import os
  
  prefix = os.environ['PREFIX']
  fn = os.path.join(prefix, 'binary-has-prefix')
  
  with open(fn, 'b') as f:
      data = f.read()
  
  print(data)
  assert prefix.encode('utf-8') in data
  
new_contents: import os
  
  prefix = os.environ['PREFIX']
  fn = os.path.join(prefix, 'binary-has-prefix')
  
  with open(fn, 'rb') as f:
      data = f.read()
  
  print(data)
  assert prefix.encode('utf-8') in data
  
subject: Fix open command