Fix generate-run-benchmark to pick the trace json file now that
run_benchmark generates a different directory structure due to the
protobuf change.

Bug: v8:10348
No-Try: true
Change-Id: I4d671071db68a7a82ec542bf41bf1d9afcdb3837
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2120590Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66868}