Add `$TEST_LOG_FILE` envar to ios_xctestrun_runner post_action

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Doc tests on :ubuntu: Ubuntu 20.04 LTScurl -sS https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?1741271680 -o bazelci.py && python3.8 bazelci.py runner --task=doc_tests
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Total Job Run Time: 1h 24m