[cleanup] Fix use of deprecated Concat in d8

Use the Isolate* version of Concat.

Bug: v8:7754
Change-Id: I3d16405032ab0690c57e2ba615cac60d8fa92464
Reviewed-on: https://chromium-review.googlesource.com/1146578Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54608}

src/d8-posix.cc

@@ -342,7 +342,7 @@ static Local<Value> GetStdout(Isolate* isolate, int child_fd,
       Local<String> addition =
           String::NewFromUtf8(isolate, buffer, NewStringType::kNormal, length)
               .ToLocalChecked();
-      accumulator = String::Concat(accumulator, addition);
+      accumulator = String::Concat(isolate, accumulator, addition);
       fullness = bytes_read + fullness - length;
       memcpy(buffer, buffer + length, fullness);
     }