fix: Use matplotlib Agg backend for headless GitHub Actions runner

Author: shlok926

q_refine/core/dashboard.py

@@ -1,3 +1,5 @@
+import matplotlib
+matplotlib.use('Agg')
 import matplotlib.pyplot as plt
 import numpy as np
 import os

q_refine_pipeline.py

@@ -1,4 +1,6 @@
 import numpy as np
+import matplotlib
+matplotlib.use('Agg')
 import matplotlib.pyplot as plt
 from qiskit_aer import AerSimulator
 from qiskit_aer.noise import NoiseModel, depolarizing_error