How can I exclude the photos that are in iCloud Photo Library from my iPhone’s iTunes backup?
After searching some more, I came across the Macworld article “How to exclude your iCloud Photo Library from an iTunes backup”. The article points out that iTunes backups are not supposed to include the iCloud Photo Library data in the first place, as documented by Apple Support:


  An iTunes backup doesn't include:
  
  
  Photos already stored in the cloud, like My Photo Stream, and iCloud Photo Library
  


The article reports that others have experienced the same problem where the backup does include the iCloud Photo Library data, and suggests disabling and re-enabling iCloud Photo Library on the iPhone.

I think I will wait for the upload from my MacBook to finish before trying this. Perhaps the problem will resolve itself after the initial upload is complete. I will edit my answer later.

Edit: The above tip may be helpful to others, though unfortunately I have to report it didn’t resolve the issue for me. Apart from disabling and re-enabling iCloud Photo Library on my iPhone, I have also tried deleting its backup in iTunes before making a new backup. But I find that the iTunes backups are still taking up a few gigabytes of additional space with iCloud Photo Library turned on compared to when it’s turned off. I’m not sure what else to try; other tips, or confirmation from others experiencing the same issue would be welcome.