fix(eval): avoid stat calls in builtins.readDir

Use entry.file_type() instead of entry.metadata().file_type() to avoid
unnecessary stat syscalls. The file_type() method uses the cached d_type
field from the readdir() syscall when available, eliminating the need
for additional filesystem access.

This matches the behavior described in https://github.com/NixOS/nix/pull/13412
where readDir should only read directory entries without stat'ing each file.

Change-Id: I8983f84a7cff28b2c892d5829b9880ed22be4389
Reviewed-on: https://cl.snix.dev/c/snix/+/30596
Reviewed-by: Florian Klink <flokli@flokli.de>
Tested-by: besadii

snix/eval/src/io.rs

@@ -152,7 +152,10 @@ impl EvalIO for StdIO {
 
         for entry in path.read_dir()? {
             let entry = entry?;
-            let file_type = entry.metadata()?.file_type();
+            // Use entry.file_type() instead of entry.metadata() to avoid stat syscalls.
+            // file_type() uses the d_type field from the readdir() syscall (when available),
+            // which is cached in the DirEntry and doesn't require additional filesystem access.
+            let file_type = entry.file_type()?;
 
             let val = if file_type.is_dir() {
                 FileType::Directory