ci: re-enable list_get_item_unchecked benchmark on free-threaded build

[davidhewitt]

This was disabled a long time ago because (IIRC) we didn't have get_item_unchecked enabled on the free-threaded build initially, once we had critical sections (I think) we re-enabled that function, but forgot the benchmark.

[codspeed-hq]

Merging this PR will degrade performance by 15.87%

❌ 1 regressed benchmark
✅ 99 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	list_get_item_unchecked	3.3 ms	3.9 ms	-15.87%

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_{Comparing davidhewitt:get-item-unchecked-bench (f55065c) with main (a6c0fdd)}