Migrant workers—from Polish farmhands working the fields of southern France to Filipino cruise-ship workers in the Caribbean—who lost their jobs because of the pandemic’s economic impact are running out of cash to send home, dealing a blow to the fragile economic health of the developing world. Tens of millions of Indians, Filipinos, Mexicans and others from developing countries working overseas sent a record $554 billion back to their home countries last year. That’s an amount greater than all foreign direct investment in…