fix(l10n): honor LC_ALL and LC_MESSAGES for locale detection (fixes #8922)

[leno23]

Summary

• Detect locale from LC_ALL, then LC_MESSAGES, then LANG (GNU precedence).
• Treat empty locale env vars as unset.
• Add regression test ensuring LC_ALL overrides LANG.

Test plan

• cargo test -p uucore test_detect_system_locale

Fixes #8922

Made with Cursor

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/expr/expr. tests/expr/expr is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/join/join. tests/join/join is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/uniq/uniq. tests/uniq/uniq is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tail/pipe-f (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)

[xtqqczze]

Suggested change

	let locale_str = locale_var("LC_ALL")
	.or_else(|| locale_var("LC_MESSAGES"))
	.or_else(|| locale_var("LANG"))
	.unwrap_or_else(|| DEFAULT_LOCALE.to_string())
	let locale_str = ["LC_ALL", "LC_MESSAGES", "LANG"]
	.into_iter()
	.find_map(|k| std::env::var(k).ok().filter(|v| !v.is_empty()))
	.as_deref()
	.unwrap_or(DEFAULT_LOCALE)

[xtqqczze]

cargo test test_env_french is failing