Make backtrace var conditional

src/main.rs

@@ -355,7 +355,9 @@ impl Future for Main {
 }
 
 fn main() {
-    env::set_var("RUST_BACKTRACE", "full");
+    if env::var("RUST_BACKTRACE").is_err() {
+        env::set_var("RUST_BACKTRACE", "full")
+    }
     let mut core = Core::new().unwrap();
     let handle = core.handle();