Limit libc::link usage to nto70 target only, not NTO OS

It seems QNX 7.0 does not support `linkat` at all (most tests were failing). Limiting to QNX 7.0 only, while using `linkat` for the future versions seems like the right path forward (tested on 7.0).

Fixes 129895

library/std/src/sys/pal/unix/fs.rs

@@ -1731,7 +1731,7 @@ pub fn link(original: &Path, link: &Path) -> io::Result<()> {
     run_path_with_cstr(original, &|original| {
         run_path_with_cstr(link, &|link| {
             cfg_if::cfg_if! {
-                if #[cfg(any(target_os = "vxworks", target_os = "redox", target_os = "android", target_os = "espidf", target_os = "horizon", target_os = "vita", target_os = "nto"))] {
+                if #[cfg(any(target_os = "vxworks", target_os = "redox", target_os = "android", target_os = "espidf", target_os = "horizon", target_os = "vita", target_env = "nto70"))] {
                     // VxWorks, Redox and ESP-IDF lack `linkat`, so use `link` instead. POSIX leaves
                     // it implementation-defined whether `link` follows symlinks, so rely on the
                     // `symlink_hard_link` test in library/std/src/fs/tests.rs to check the behavior.