src: fix duplication of externalized builtin code

The `else` here was missing, causing us to append the file to its
existing `source` already in the map.

Refs: https://github.com/nodejs/node/pull/45942
PR-URL: https://github.com/nodejs/node/pull/47079
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

src/node_builtins.cc

@@ -224,8 +224,7 @@ void BuiltinLoader::AddExternalizedBuiltin(const char* id,
     auto it = externalized_builtin_sources.find(id);
     if (it != externalized_builtin_sources.end()) {
       source = it->second;
-    }
-    {
+    } else {
       int r = ReadFileSync(&source, filename);
       if (r != 0) {
         fprintf(stderr,