行こう with a reading of ゆこう


In one of the opening songs for the TV show ちびまる子かちゃん the singer says

> 友達 探しに 行こうよ

<https://i.gyazo.com/a349820aaf28ca1f13c1c772d889ac2e.png>

but 行こう is pronounced ゆこう instead of いこう

Why is 行こう being pronounced this way?

