KEY TAKEAWAYSBillionaire investor Bill Ackman built new stakes in Nike during the second quarter, according to a regulatory filing, helping send shares of the sneaker giant up around 4% in premarket trading.The SEC filing showed Ackman's Pershing Square Capital Management had built a stake of more than 3 million shares.Nike has been struggling with falling sales amid a slowing Chinese economy and rising competition from upstarts like Hoka. Billionaire investor Bill Ackman built new stakes in Nike (NKE) duri ...