Fix build of ruff_benchmark on NixOS

[MichaReiser]

Summary

See #13299 (comment)

This PR fixes the ruff_benchmark build on NixOS by not relying on the unprefixed_malloc_on_supported_platforms feature and instead export the conf as _rjem_malloc_conf

[codspeed-hq]

CodSpeed Performance Report

Merging #13366 will improve performances by 4.1%

_{Comparing micha/fix-jemalloc-conf-nix (ae9987e) with main (1365b08)}

Summary

⚡ 1 improvements
✅ 31 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	micha/fix-jemalloc-conf-nix	Change
⚡	linter/default-rules[pydantic/types.py]	1.9 ms	1.8 ms	+4.1%

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

Hmm okay, now the conf variable isn't picked up anymore. Perfect

[MichaReiser]

Let's see if this works. I don't see any new decay calls. The perf differences are only re_alloc.

[GaetanLepage]

Backporting this patch to 0.6.5 is working :)

[MichaReiser]

Backporting this patch to 0.6.5 is working :)

Nice. Thanks for confirming. Let's hope our benchmarks don't regress to being flaky again.