Starbucks' Nashville expansion is part of a broader fast-food migration to the Southern US. Bloomberg/Bloomberg via Getty Images Starbucks and In-N-Out have recently announced plans to build corporate offices in Tennessee. The moves are part of a broader southern expansion across the fast food industry. New and legacy brands are expanding in the South due to demand, lower costs, and easier hiring. Starbucks is planting a corporate flag in Nashville. In-N-Out is building an Eastern hub in nearby Franklin, Tennessee. And across the South, a long list of fast-food names — like Whataburger, Cava, and Jersey Mike's — are expanding. From Texas to Florida, burgeoning and legacy fast-food companies are clustering their offices and growth bets in the region, chasing lower operating costs, easier hiring, and a customer base that's clamoring for more options. Recruiters and consultants who work across the restaurant industry say the move has been building for years, but is now accelerating.…