build: use "selinux" instead of "feat_selinux" feature name

[xtqqczze]

Do not specify feat_selinux in individual crates; it is now only defined at the workspace level.

cc: @cakebaker

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!

[oech3]

coreutils/README.md

Lines 123 to 126 in bd908f0

	To build SELinux-specific features, including `chcon` and `runcon`, ensure that `libselinux`
	and `libclang` are installed on your system. Then, run the following command:
	```
	cargo build --release --features unix,feat_selinux

[oech3]

why test only?

[xtqqczze]

@sylvestre could you please review?

[codspeed-hq]

Merging this PR will degrade performance by 4.53%

❌ 2 regressed benchmarks
✅ 310 untouched benchmarks
⏩ 57 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	ls_recursive_balanced_tree[(6, 4, 15)]	49.6 ms	52.3 ms	-5.23%
❌	Simulation	ls_recursive_wide_tree[(10000, 1000)]	33 ms	34.3 ms	-3.83%

Tip

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

---

_{Comparing xtqqczze:selinux (b5e0a52) with main (542e7af)}

Footnotes

1. 57 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. ↩