use global $wp_query->found_posts instead of creating a new WP_Query to count search results in trackSiteSearch mode, for better performance

[theodejager]

When "Track search: Use Matomo's advanced Site Search Analytics feature" is selected, an extra/new WP_Query is ran to count the number of results and push them to Matomo. This can really slow down searches.

Using the already available global $wp_query object avoids having to run an extra query.