fix(deps): refactor rustix/nix package configuration

[xtqqczze]

Refactor rustix and nix dependency configuration to use explicit, target-specific features.

Benefits

• Avoid enabling unused features on platforms where they aren't needed.
• Keep platform-specific dependencies scoped to the relevant targets.
• Make the Cargo configuration easier to understand and maintain.
• Reduce unnecessary dependency features and cross-platform coupling.

[codspeed-hq]

Merging this PR will not alter performance

✅ 361 untouched benchmarks
⏩ 50 skipped benchmarks¹

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_{Comparing xtqqczze:deps/rustix (f0222eb) with main (dfb953d)}

Footnotes

1. 50 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout-group (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)

[cakebaker]

Thanks!