Add a lock to prevent races between {WasmImportWrapperCache::Get} and
the cache modification scope.

R=clemensb@chromium.org

Change-Id: Ife281c127c765d3ca57f58c975e15a76833983ca
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1965588
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65458}