tools: fix use-after-free mkcodecache warning

Call `v8::Platform::ShutdownPlatform()` to fix a Coverity warning about
the `v8::Platform` instance being deleted when it's still in use.

PR-URL: https://github.com/nodejs/node/pull/27332


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>