[Release] Use with clause to make sure result json file is closed properly

[jjyao]

Why are these changes needed?

If the file is not closed correctly, the data might be lost.

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[can-anyscale]

Looking good; let's confirm with the release test run

[jjyao]

The issue is fixed

[INFO 2024-07-09 23:15:19,390] log.py: 41  Observed the following results:
  |  
  | broadcast_time = 80.23293005799997
  | object_size = 1073741824
  | num_nodes = 50
  | success = 1
  | perf_metrics = [{'perf_metric_name': 'time_to_broadcast_1073741824_bytes_to_50_nodes', 'perf_metric_value': 80.23293005799997, 'perf_metric_type': 'LATENCY'}]
 

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