After building Method, the sustainable soap brand, into a $100 million-plus business, cofounder Adam Lowry probably could have retired. Instead, he tackled a new problem: the unsustainability of milk. Dairy has a massive carbon footprint, and a single gallon of milk takes 1,000 gallons of water to produce. Plant-based alternatives, from almond milk to coconut milk, are gaining in popularity, but they have issues of their own. So Ripple is using another unusual ingredient—peas—instead. “It’s…