What’s better for the environment, Green Works or Seventh Generation, or maybe Method? Who’s to say? How about Wal-Mart. The world’s largest retailer is making good on its threat to develop a rating system to measure the sustainability of its supply chain. Wal-Mart has been working on its”sustainability index” for more than a year, and observers say it will take another year or two for labels to appear on products. For now, Wal-Mart intends to announce the sustaina...