fix: redirect cargo build output to stderr to avoid polluting tool stdout

Redirect cargo build output (messages, warnings) to stderr instead of stdout. This ensures that when redirecting or piping the output of cargo-binutils tools, only the tool's output appears on stdout, not cargo/rustc messages.

Author: nhpupu

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Fixed
+
+- Redirected cargo build output to stderr instead of stdout. This prevents cargo/rustc messages and warnings from appearing in the stdout of cargo-binutils tools, making it easier to pipe or redirect tool output cleanly.
+
 ## [v0.4.0] - 2025-08-26
 
 ### Added

src/lib.rs

@@ -449,7 +449,7 @@ fn cargo_build(matches: &ArgMatches, metadata: &Metadata) -> Result<Option<Artif
             Message::CompilerMessage(msg) => {
                 if !quiet || verbose > 1 {
                     if let Some(rendered) = msg.message.rendered {
-                        print!("{rendered}");
+                        eprint!("{rendered}");
                     }
                 }
             }