It's already biased towards the face that is facing up to start. With practice or by accident, you can toss it so that it wobbles but doesn't actually flip over. A casual observer cannot tell the difference.
TODO: Build a robot with precise enough control that it can flip a coin (properly flipping, not just wobbling) and then catch it in a desired state (either heads or tails) with substantial accuracy.
That reminds me of the rock-paper-scissors robot from a while ago that always "wins" by watching the human hand to see what sign they are throwing, then throwing the countersign faster. It is so fast that unless you are watching it on high-speed film, you can't tell that it is cheating.