Fix color for rustdoc error messages

Due to a change in tracing-subscriber where it escapes ANSI codes, the
output from rustdoc was changed to be escaped instead of colored. This
fixes the issue by just printing to the console directly.

This does not have a test because it is difficult to emulate a terminal
in the testsuite, and I don't feel like adding color overrides at the
moment.

Ref:
- https://github.com/rust-lang/mdBook/pull/2829 -- Switched to tracing
- https://github.com/advisories/GHSA-xwfj-jgwm-7wp5 -- CVE-2025-58160
- https://github.com/tokio-rs/tracing/issues/3378 -- Discussion of
  supporting color codes.

Eventually mdbook's primary output should migrate away from using log
messages, and instead use a structured console system. However, that's a
ways off.

crates/mdbook-driver/src/mdbook.rs

@@ -20,7 +20,7 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 use tempfile::Builder as TempFileBuilder;
 use topological_sort::TopologicalSort;
-use tracing::{debug, error, info, trace, warn};
+use tracing::{debug, info, trace, warn};
 
 #[cfg(test)]
 mod tests;
@@ -329,8 +329,8 @@ impl MDBook {
 
                 if !output.status.success() {
                     failed = true;
-                    error!(
+                    eprintln!(
-                        "rustdoc returned an error:\n\
+                        "ERROR rustdoc returned an error:\n\
                         \n--- stdout\n{}\n--- stderr\n{}",
                         String::from_utf8_lossy(&output.stdout),
                         String::from_utf8_lossy(&output.stderr)