Apply prefer-const lint rule

lib/logger.js

@@ -221,8 +221,8 @@ function formatSimple(details, systemd) {
 
   let meta = "";
 
-  let kv = [];
-  for (let k of keys)
+  const kv = [];
+  for (const k of keys)
     if (!skip[k]) kv.push(`${k}=${JSON.stringify(details[k])}`);
 
   if (kv.length) meta = `; ${kv.join(" ")}`;