Networks and Sites: Ensure the site ID in `wp_insert_site()` is still correct...

Networks and Sites: Ensure the site ID in `wp_insert_site()` is still correct after calling `clean_blog_cache()`.

By saving `$wpdb->insert_id` to a variable, we make sure the ID does not change if another database insert is performed on the `clean_site_cache` action.

Props trepmal.
Fixes #50324.
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