Add track_caller to builder_helper::output

If something goes wrong here, showing `fn output` is unhelpful. Show
where the command is being run instead.

src/build_helper/lib.rs

@@ -130,6 +130,7 @@ pub fn make(host: &str) -> PathBuf {
     }
 }
 
+#[track_caller]
 pub fn output(cmd: &mut Command) -> String {
     let output = match cmd.stderr(Stdio::inherit()).output() {
         Ok(status) => status,