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/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"));
+}