satellite/payments: De-shadow customer page var in stripe mock

Fixes infinite loop when there are more records than the limit.

Change-Id: Ib61a8fcad6d08f4e0f1c5715d710acec607004b3

satellite/payments/stripecoinpayments/stripemock.go

@@ -155,7 +155,7 @@ func (m *mockCustomers) repopulate() error {
 		}
 
 		for cusPage.Next {
-			cusPage, err := m.customersDB.List(ctx, cusPage.NextOffset, limit, time.Now())
+			cusPage, err = m.customersDB.List(ctx, cusPage.NextOffset, limit, time.Now())
 			if err != nil {
 				return err
 			}