main: Log to stderr instead of stdout

Currently we can't use logging in paths like niri msg that have meaningful
stdout. Logging to stderr makes that possible. Even if we don't want to log
anything in niri msg code paths, it's easy to have something accidentally log.

src/main.rs

@@ -52,6 +52,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     let env_filter = EnvFilter::builder().parse_lossy(directives);
     tracing_subscriber::fmt()
         .compact()
+        .with_writer(io::stderr)
         .with_env_filter(env_filter)
         .init();