ci: lower Codspeed CUDA runner CPU count

[0ax1]

Summary

• keep CUDA Codspeed benchmarks on g5 runners
• request 8 CPU cores for the GPU runner

[codspeed-hq]

Merging this PR will degrade performance by 16.51%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

❌ 1 regressed benchmark
✅ 1265 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_varbinview_opt_canonical_into[(1000, 10)]	187.9 µs	225 µs	-16.51%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing codspeed-g5-cpu-8 (9b7fe30) with develop (87273dc)}