fix: add comma separator and format macro

src/server/routes/search.rs

@@ -138,7 +138,7 @@ async fn results(
 
         // We sort the list of engine so the cache keys will match between users. The cookie's list of engines is unordered.
         engines.sort();
-        cache_key = cache_key + &(engines.join(""));
+        cache_key = format!("{}{}", cache_key, engines.join(","));
     } else {
         let mut engines: Vec<String> = config
             .upstream_search_engines
@@ -148,7 +148,7 @@ async fn results(
             .collect();
 
         engines.sort();
-        cache_key = cache_key + &(engines.join(""));
+        cache_key = format!("{}{}", cache_key, engines.join(","));
     }
 
     // fetch the cached results json.