fix: avoid mutating summarize_args list in summarize()

[norus]

Summary

Fix a list mutation bug in scripts/performance/benchmark_utils.py.

Closes #10121

Problem

summarize() called .extend() on the shared summarize_args list before the second subprocess.check_call(). This mutates the list in-place, making the function non-idempotent: any repeated call accumulates duplicate --output-format json flags.

# Before (buggy)
with open(os.path.join(summary_dir, 'summary.json'), 'wb') as f:
    summarize_args.extend(['--output-format', 'json'])  # mutates original
    subprocess.check_call(summarize_args, stdout=f)

Fix

Construct a new list for the JSON invocation, leaving the original untouched.

# After
with open(os.path.join(summary_dir, 'summary.json'), 'wb') as f:
    json_args = summarize_args + ['--output-format', 'json']
    subprocess.check_call(json_args, stdout=f)

Testing

Verified locally that summarize_args is not mutated after the call, and the JSON summary subprocess receives exactly the expected arguments.

[norus]

Closing as duplicate of #10127 which was opened first with the same fix.