gem5: M5_OVERRIDE_PY_SOURCE

run

@@ -102,6 +102,8 @@ def main(args, extra_args=None):
         if do_trace:
             gem5_exe_args.append('--debug-flags={}'.format(trace_type))
         extra_env['M5_PATH'] = common.gem5_system_dir
+        # https://stackoverflow.com/questions/52312070/how-to-modify-a-file-under-src-python-and-run-it-without-rebuilding-in-gem5/52312071#52312071
+        extra_env['M5_OVERRIDE_PY_SOURCE'] = 'true'
         cmd = (
             debug_vm +
             [