Confirmation bias is defined as (OED): "The tendency to interpret new evidence as confirmation of one’s existing beliefs or theories."

The adversarial justice system is expressly designed to avoid confirmation bias. That's the whole point of the "adversarial" part. While the prosecution might be prone to selectively interpreting the facts to confirm its own beliefs, the defense counsel is tasked with poking holes in that interpretation. That's why defense counsel has the independent opportunity to present evidence, and to cross-examine the prosecution's witnesses to poke holes in their story.