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/date/resolution (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cut/cut-huge-range 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?