The advice sounds tidy: skip the $28 lunch, invest the money, retire richer. Kevin O’Leary has been making a version of that pitch for years. Bernie Sanders says the people handing out that advice have no idea what a $70,000 salary actually buys anymore. O’Leary’s argument, delivered on “The Diary of a CEO” podcast, came down to compound interest: a worker earning $70,000 who drops $28 on lunch is torching money that could have grown for decades…