src: lock the thread properly in snapshot builder

Otherwise it can crash DCHECK when V8 expects that at least someone
is locking the current thread.

PR-URL: #56327
Fixes: nodejs/node-v8#294
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: joyeecheung

src/node_snapshotable.cc

@@ -962,6 +962,8 @@ ExitCode BuildSnapshotWithoutCodeCache(
   }
 
   Isolate* isolate = setup->isolate();
+  v8::Locker locker(isolate);
+
   {
     HandleScope scope(isolate);
     TryCatch bootstrapCatch(isolate);