driver: Only output ANSI if connected to a terminal

See #78435 for more.

compiler/rustc_driver/src/lib.rs

@@ -1286,7 +1286,7 @@ pub fn init_env_logger(env: &str) {
     let filter = tracing_subscriber::EnvFilter::from_env(env);
     let layer = tracing_tree::HierarchicalLayer::default()
         .with_indent_lines(true)
-        .with_ansi(true)
+        .with_ansi(stdout_isatty())
         .with_targets(true)
         .with_thread_ids(true)
         .with_thread_names(true)