save command: Don't use BufWriter to write external strem to a file (#9377)

# Description
Fixes: #9293

The problem is caused by `save` makes a `BufferWriter` for output file,
when external commands redirect it's output to a file, the content is
bufferred first...

To fix the issue, I'd like to introduce a `--no-buf` flag for `save`
command, and it's only used in redirection scenario.

Sorry it's hard to test against it in test, because it requires external
command to sleep or pause...

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---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

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

@@ -6,7 +6,7 @@ use nu_protocol::{
     Type, Value,
 };
 use std::fs::File;
-use std::io::{BufWriter, Write};
+use std::io::Write;
 use std::path::Path;
 use std::thread;
 
@@ -349,7 +349,9 @@ fn stream_to_file(
     span: Span,
     progress: bool,
 ) -> Result<PipelineData, ShellError> {
-    let mut writer = BufWriter::new(file);
+    // https://github.com/nushell/nushell/pull/9377 contains the reason
+    // for not using BufWriter<File>
+    let mut writer = file;
 
     let mut bytes_processed: u64 = 0;
     let bytes_processed_p = &mut bytes_processed;