Avoid data race by only setting FLAG_local_heaps to true if not
already enabled.

Bug: v8:10315
Change-Id: Ib562b6d525448f5c088da39bf60928debd97db43
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2426610Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70115}