introduce IS_POSIXLY_CORRECT static

[xtqqczze]

Follow-up #11853

[xtqqczze]

Since we control the entrypoint, we could use unsafe mutable static instead:

static mut IS_POSIXLY_CORRECT: bool = false;

pub fn uu_main() {
    unsafe {
        IS_POSIXLY_CORRECT = env::var_os("POSIXLY_CORRECT").is_some();
    }
}

fn is_posixly_correct() -> bool {
    unsafe { IS_POSIXLY_CORRECT }
}

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/expand/bounded-memory is now passing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 309 untouched benchmarks
⏩ 46 skipped benchmarks¹

---

_{Comparing xtqqczze:static-posixly-correct2 (5652044) with main (8cc048b)}

Footnotes

1. 46 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. ↩

[cakebaker]

Hm, what's the benefit of these changes? For example, in mktemp.rs there is a single check for the POSIXLY_CORRECT env variable in uumain, and using a global LazyLock seems overkill. But maybe I'm missing something...