Silly question, I know.
It will not have anything like the flavor.
To get a flavor closer to buttermilk, you can squeeze a lemon into the plain milk. Not exact but closer!
May I respectfully suggest you bake a different pie this time? But do try a buttermilk pie when you can. They are delicious.
It worked ok! A little different flavor, but the texture was fine. I added cinnamon for a fall flair, and it was a hit!
Buttermilk has a wonderful tang, which is what makes it so good in a pie. Milk that hasn't been cultured just won't have the flavor. It will be a custard pie, not a buttermilk pie! It will be good, just not the same.