A young boy comes with a history of fever and cough since two weeks. He has reduced appetite and weight loss. He is also a known case of congenital heart disease, for which he has taken prophylaxis with penicillin. Sputum examination is normal; however, bronchoscopic lavage cultured on sheep blood agar showed evidence of gram-positive, aerobic, weakly acid-fast branching filaments. What is the likely diagnosis?