chore(bench): disable mimalloc purge delay for codspeed cpu simulation

[stormslowly]

Why

Mimalloc delayed purging can call clock_gettime during CPU simulation measurements, leaking allocator timing noise into CodSpeed stage benchmarks. Setting MIMALLOC_PURGE_DELAY=-1 on the simulation run keeps that noise out while leaving the memory run unchanged.

Ports web-infra-dev/rspack#14321 to this repo. Only the CI workflow change applies here — there is no bench:rust:local script or xtask/benchmark/README.md to mirror.

This repo uses mimalloc as the global allocator in benches/resolver.rs, so the same purge-noise concern applies.

[Copilot]

Pull request overview

This PR updates the benchmark CI workflow to reduce allocator-related timing noise during CodSpeed CPU (simulation) benchmarks by disabling mimalloc’s delayed purge behavior for that run only.

Changes:

• Set MIMALLOC_PURGE_DELAY=-1 for the CodSpeed CPU benchmark step (mode: simulation) to avoid clock_gettime noise from delayed purging.
• Leave the memory benchmark step unchanged to preserve existing behavior.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds the environment variable MIMALLOC_PURGE_DELAY: "-1" to the CPU benchmark job step in the GitHub Actions workflow. This setting disables memory purge delay for the miMalloc allocator during benchmark simulation mode. The change affects only the benchmark workflow configuration and does not modify any workflow logic, steps, triggers, or permissions.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description is directly related to the changeset, explaining the rationale for adding the MIMALLOC_PURGE_DELAY environment variable and its impact on benchmark measurements.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title accurately describes the main change: adding MIMALLOC_PURGE_DELAY=-1 to disable mimalloc purge delay for codspeed CPU simulation in the benchmark workflow.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/codspeed-mimalloc-purge-delay

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[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

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_{Comparing ci/codspeed-mimalloc-purge-delay (21f4cd4) with main (02dbd01)}