Aug 26 (Reuters) - Dollar Tree Inc (DLTR.O) cut its full-year profit forecast on Thursday as the discount retailer joined rival Dollar General Corp (DG.N) in flagging that surging supply chain costs were pinching its margins. The company said it now expects fiscal 2021 earnings per share of $5.40 to $5.60, compared with its prior forecast of $5.80 to $6.05.