As a home-share host who shares my kitchen with guests, this has never been an issue for me, as I use cream in my coffee as well, so there is always some
in the fridge.
I agree with you on wasted food and environmental impact.
What I might do in your situation is have a saved message which you include in questions or info sent to guests like “While I supply coffee beans, a grinder and a French press, I found the milk I was leaving for guests often went bad as they didn’t use it. I would be happy to provide milk or cream for coffee if you let me know that you use it so it isn’t just wasted food.”
If they start demanding alternatives, like soy milk, almond milk, etc, as KKC said, I wouldn’t try to cater to that- you’d have to open a fresh litre carton and the rest would go bad unless you use it yourself.