don't allow save command to save both stdout and stderr to the same file (#9368)

# Description
It's not a good idea to save `stdout` and `stderr` to the same file from
`save` command directly.

Because it saves `stdout` and `stderr` in different thread, which leads
to in-consistent output. As replace, we can use `o+e` redirection to fix
the issue

# User-Facing Changes
```
❯ do -i { "aa" } | save foo.txt -e foo.txt
Error:   × input and stderr input to same file
   ╭─[entry #3:1:1]
 1 │ do -i { "aa" } | save foo.txt -e foo.txt
   ·                                  ───┬───
   ·                                     ╰── can't save both input and stderr input to the same file
   ╰────
  help: you should use `o+e> file` instead
```

# Tests + Formatting
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crates/nu-command/src/filesystem/save.rs

@@ -301,18 +301,6 @@ fn open_file(path: &Path, span: Span, append: bool) -> Result<File, ShellError>
     })
 }
 
-fn clone_file(file: &File, span: Span) -> Result<File, ShellError> {
-    file.try_clone().map_err(|err| {
-        ShellError::GenericError(
-            "Permission denied".into(),
-            err.to_string(),
-            Some(span),
-            None,
-            Vec::new(),
-        )
-    })
-}
-
 /// Get output file and optional stderr file
 fn get_files(
     path: &Spanned<String>,
@@ -333,7 +321,13 @@ fn get_files(
     let stderr_file = stderr_path_and_span
         .map(|(stderr_path, stderr_path_span)| {
             if path == stderr_path {
-                clone_file(&file, stderr_path_span)
+                Err(ShellError::GenericError(
+                    "input and stderr input to same file".to_string(),
+                    "can't save both input and stderr input to the same file".to_string(),
+                    Some(stderr_path_span),
+                    Some("you should use `o+e> file` instead".to_string()),
+                    vec![],
+                ))
             } else {
                 open_file(stderr_path, stderr_path_span, append)
             }

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

@@ -84,13 +84,11 @@ fn save_append_will_not_overwrite_content() {
 }
 
 #[test]
-fn save_stderr_and_stdout_to_same_file() {
+fn save_stderr_and_stdout_to_afame_file() {
     Playground::setup("save_test_5", |dirs, sandbox| {
         sandbox.with_files(vec![]);
 
-        let expected_file = dirs.test().join("new-file.txt");
-
-        nu!(
+        let actual = nu!(
             cwd: dirs.root(),
             r#"
             let-env FOO = "bar";
@@ -98,11 +96,9 @@ fn save_stderr_and_stdout_to_same_file() {
             do -c {nu -c 'nu --testbin echo_env FOO; nu --testbin echo_env_stderr BAZ'} | save -r save_test_5/new-file.txt --stderr save_test_5/new-file.txt
             "#,
         );
-
-        let actual = file_contents(expected_file);
-        println!("{}, {}", actual, actual.contains("ZZZ"));
-        assert!(actual.contains("bar"));
-        assert!(actual.contains("ZZZ"));
+        assert!(actual
+            .err
+            .contains("can't save both input and stderr input to the same file"));
     })
 }