Despite increasing scrutiny from state regulators, many Massachusetts hospital workers are failing to get flu vaccinations, exposing patients to a heightened risk of infections, according to a report released Wednesday by the Department of Public Health. Roughly 81 percent of employees at the state’s acute care hospitals received flu shots during the most recent season. The percentages increased from a statewide average of about 71 percent during the previous season but fell short of regulators’ goal of 90 percent.