Change params on get_site_by_path

[spacedmonkey]

In get_site_by_path, it calls get_sites. In most cases in domain and path only have one elemeent in their arrays. To improve performance, changing it to domain instead of domain__in will increase likelyhood it will hit an existing cache.

[SergeyBiryukov]

In 42698:

Networks and Sites: In get_site_by_path(), use single domain and path parameters if there's only one item to look for.

This should increase the chances of WP_Site_Query::get_sites() hitting an existing cache.

Props spacedmonkey.
Fixes #42299.