Fix benchmark workflow package identity

[p-x9]

Summary

• check out both benchmark refs under directories named MachOKit
• update candidate and baseline root variables to match the nested checkout paths
• invoke the reusable benchmark script from its new candidate path

Root cause

Benchmarks/Package.swift declares the repository with .package(path: "..") and references the product with package: "MachOKit". SwiftPM derives the identity of that local dependency from its directory name. The workflow checked the repository out as candidate and baseline, so the candidate dependency was identified as candidate instead of MachOKit and dependency resolution failed.

Impact

Manual single-ref and comparison benchmark runs can now resolve the local MachOKit package in GitHub Actions without changing the benchmark package manifest.

Validation

• parsed .github/workflows/benchmark.yml as YAML
• ran bash -n scripts/benchmark.sh
• verified all candidate and baseline roots use paths ending in MachOKit
• ran git diff --check

The workflow still needs to be re-run on GitHub Actions to verify the hosted-runner execution end to end.

[Copilot]

Pull request overview

This PR fixes the GitHub Actions benchmark workflow by ensuring SwiftPM derives the correct local package identity (MachOKit) during benchmark builds, aligning the workflow checkout directory structure with how Benchmarks/Package.swift references the root package via .package(path: "..").

Changes:

• Check out both candidate and baseline refs into directories named MachOKit (nested under candidate/ and baseline/).
• Update CANDIDATE_ROOT and BASELINE_ROOT to point at the nested .../MachOKit directories.
• Invoke the benchmark script using the new nested candidate path.

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