David Zalik, founder and CEO of GreenSky. Chris Hamilton | GreenSky Goldman Sachs, the 153-year old investment bank, is now officially in the home improvement loan business as it continues a push into the finances of ordinary Americans. The bank expects to add one million customers a year to its budding Marcus retail division through the acquisition of GreenSky, announced in September at a $2.2 billion price tag, Goldman executives said in their first interview after the deal closed Tuesday. GreenSky is an Atlanta-based buy-now, pay-later fintech firm that focuses on construction loans with an average $10,000 size. “It a great acquisition engine because we expect to bring a million…