fix wrong error msg of save command on windows (#14699)

fixes #14664 

# Description

Now,

```nu
"aaa" | save -f ..
```

returns correct error message on windows.

Note that the fix introduces a TOCTOU problem, which only effects the
error message. It won't break any workload.

# User-Facing Changes

The fix won't break any workload.

# Tests + Formatting

I have run tests **only on windows**.

# After Submitting

The fix doesn't need to change documentation.

crates/nu-command/src/filesystem/save.rs

@@ -422,7 +422,22 @@ fn prepare_path(
 fn open_file(path: &Path, span: Span, append: bool) -> Result<File, ShellError> {
     let file = match (append, path.exists()) {
         (true, true) => std::fs::OpenOptions::new().append(true).open(path),
-        _ => std::fs::File::create(path),
+        _ => {
+            // This is a temporary solution until `std::fs::File::create` is fixed on Windows (rust-lang/rust#134893)
+            // A TOCTOU problem exists here, which may cause wrong error message to be shown
+            #[cfg(target_os = "windows")]
+            if path.is_dir() {
+                // It should be `io::ErrorKind::IsADirectory` but it's not available in stable yet (1.83)
+                Err(io::Error::new(
+                    io::ErrorKind::Unsupported,
+                    "Is a directory (os error 21)",
+                ))
+            } else {
+                std::fs::File::create(path)
+            }
+            #[cfg(not(target_os = "windows"))]
+            std::fs::File::create(path)
+        }
     };
 
     file.map_err(|e| ShellError::GenericError {

crates/nu-command/tests/commands/save.rs

@@ -525,3 +525,12 @@ fn parent_redirection_doesnt_affect_save() {
         assert_eq!(actual.trim_end(), "");
     })
 }
+
+#[test]
+fn force_save_to_dir() {
+    let actual = nu!(cwd: "crates/nu-command/tests/commands", r#"
+        "aaa" | save -f ..
+        "#);
+
+    assert!(actual.err.contains("Is a directory"));
+}