[proxy] Respect the magic RUST_LOG env variable

Usage: `RUST_LOG=trace proxy ...`

proxy/src/main.rs

@@ -43,6 +43,7 @@ async fn flatten_err(
 async fn main() -> anyhow::Result<()> {
     tracing_subscriber::fmt()
         .with_ansi(atty::is(atty::Stream::Stdout))
+        .with_env_filter(tracing_subscriber::EnvFilter::from_default_env())
         .with_target(false)
         .init();