Toss the cucumber and scallion slices in a small bowl with the lemon juice, sesame oil and a pinch of salt. Lightly toast the bagels. Meanwhile, melt the butter in a medium saucepan over medium heat. Add the eggs and a pinch of salt and cook, stirring, until almost set, about 2 minutes. Stir in the salmon, cream cheese and Sriracha and cook until the eggs are set, about 1 more minute. Divide the scrambled egg mixture between the bagels and top with the cucumber salad.