Print stderr streams to stderr in pipeline_data::print_if_stream() (#11929)

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Related to #11928 - `tee --stderr` doesn't really work as expected
without it

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Print stderr streams to stderr in `pipeline_data::print_if_stream()`

This corrects unexpected behavior if a stream from an external program
is transformed while still preserving its stderr output. Before this
change, that output is just drained and discarded. Worse, it's drained
to a buffer, which could be really slow and memory hungry if there's a
lot of output on stderr.

This is needed to make `tee --stderr` function in a non-surprising way.
See #11928

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A script that was erroneously not producing stderr output before might
now, but I can't think of a lot of examples of an external stream being
transformed without being converted.

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crates/nu-protocol/src/pipeline_data.rs

@@ -4,9 +4,9 @@ use crate::{
     format_error, Config, ListStream, RawStream, ShellError, Span, Value,
 };
 use nu_utils::{stderr_write_all_and_flush, stdout_write_all_and_flush};
-use std::path::PathBuf;
 use std::sync::{atomic::AtomicBool, Arc};
 use std::thread;
+use std::{io::Write, path::PathBuf};
 
 const LINE_ENDING_PATTERN: &[char] = &['\r', '\n'];
 
@@ -846,13 +846,29 @@ pub fn print_if_stream(
     to_stderr: bool,
     exit_code: Option<ListStream>,
 ) -> Result<i64, ShellError> {
-    // NOTE: currently we don't need anything from stderr
-    // so we just consume and throw away `stderr_stream` to make sure the pipe doesn't fill up
-
     if let Some(stderr_stream) = stderr_stream {
+        // Write stderr to our stderr, if it's present
         thread::Builder::new()
             .name("stderr consumer".to_string())
-            .spawn(move || stderr_stream.into_bytes())
+            .spawn(move || {
+                let RawStream {
+                    stream,
+                    leftover,
+                    ctrlc,
+                    ..
+                } = stderr_stream;
+                let mut stderr = std::io::stderr();
+                let _ = stderr.write_all(&leftover);
+                drop(leftover);
+                for bytes in stream {
+                    if nu_utils::ctrl_c::was_pressed(&ctrlc) {
+                        break;
+                    }
+                    if let Ok(bytes) = bytes {
+                        let _ = stderr.write_all(&bytes);
+                    }
+                }
+            })
             .expect("could not create thread");
     }