Bake provenance JSON into images and auto-populate post_results.py args

[misiugodfrey]

Summary

• Writes /opt/velox-testing/provenance.json into each Presto image at build time (both native_build.dockerfile and provenance_labels.dockerfile), so run_context.py can read it from the container filesystem in both Docker and SLURM/Enroot environments (Enroot .sqsh images cannot expose OCI metadata like labels, but the baked-in file is accessible via the normal filesystem)
• run_context.py reads the provenance file and merges presto_sha/branch/repo and velox_sha/branch/repo into the benchmark context dict written to benchmark_result.json
• post_results.py auto-populates --velox-branch, --velox-repo, --presto-branch, --presto-repo from the benchmark_result.json context; CLI args still take precedence; older result files without provenance fields are handled gracefully
• presto-build.yml: replaces the labels: input on the deps and coordinator build steps with a second provenance_labels.dockerfile wrapper step — buildx labels: applies OCI metadata only and does not execute RUN steps, so the provenance file would not have been written via that path

Test plan

• Build a new native worker image: presto/scripts/start_native_cpu_presto.sh
• Confirm provenance file exists: docker run --rm presto-native-worker-cpu:$USER cat /opt/velox-testing/provenance.json
• Run a short benchmark: presto/scripts/run_benchmark.sh -b tpch -s bench_sf1
• Confirm benchmark_result.json context has velox_branch, velox_repo, presto_branch, presto_repo populated
• Run post_results.py --dry-run without branch/repo args; confirm payload engine_config shows values auto-populated from the result file
• Run with --velox-branch override; confirm CLI value wins over the label
• Test with an older result file (no provenance in context); confirm the fields are absent from engine_config without error
• Trigger a CI build to produce images via the updated presto-build.yml; pull the resulting image into a SLURM/Enroot context and run a quick benchmark to confirm provenance fields are populated via the filesystem path