Fix run stop check

[maanug-nv]

Existing check will pass on lines such as:

:::MLLOG {"namespace": "", "time_ms": 1684006089927, "event_type": "POINT_IN_TIME", "key": "eval_accuracy", "value": {0}, "metadata": {"file": "check_run_stop.py", "lineno": 411, "epoch_num": 16}}

This is line contains 'run_stop' string, but this is not a run_stop event, in which case 'run_stop' is the key.

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[maanug-nv]

@pgmpablo157321 fyi. We can discuss this tomorrow.

cc @ShriyaPalsamudram