Put the oil, garlic, and jalapeno in a small skillet over medium heat and cook, stirring, until the garlic is lightly browned and fragrant, about 5 minutes. Put the sugar in a bowl and stir in the garlic mixture; cool. Add the lime juice and fish sauce and stir to dissolve the sugar completely. Add the carrots, scallion, and cilantro and toss. Serve immediately or refrigerate in a tightly sealed container for up to 3 days.