fix(client): prevent unusual commas in iframe (#44477)

* fix: Moved src files in the head

* fix: Removed excess indents

Co-authored-by: Canopius <canopius@DESKTOP-KJC5QOG.localdomain>

client/src/templates/Challenges/rechallenge/builders.js

@@ -91,7 +91,7 @@ A required file can not have both a src and a link: src = ${src}, link = ${link}
       }
       return '';
     })
-    .reduce((head, element) => head.concat(element), []);
+    .join('\n');
 
   const indexHtml = findIndexHtml(challengeFiles);