Can't create new blog in multisite if database insert happens on clean_site_cache

[trepmal]

In multisite, you cannot create a new blog if there is a database insert happening on the clean_site_cache action.

Introduced in r45910

https://core.trac.wordpress.org/browser/trunk/src/wp-includes/ms-site.php#L68-L74

On L68, the blog data is inserted into the blogs table. The new blog ID is in $wpdb->insert_id. This ID is then needed shortly after at L74 to get the WP_Site object.
However, clean_blog_cache() has an action hook. If something hooked in there does a database insert, the value of $wpdb->insert_id changes. This is likely to cause a Could not retrieve site data. but there are some other scenarios:

• If $wpdb->insert_id does not match any blog ID: Error: Could not retrieve site data.

• If $wpdb->insert_id matches a valid blog: potentially perform actions against the wrong blog. Anything run on wp_initialize_site will run against the preexisting blog, not the newly created one.

• If $wpdb->insert_id matches a half-installed blog (such as from case 1): The original blog will be fully installed since L74 now returns truthy. However, there will be some mixed values (home/siteurl will match wp_blogs values of older site, blogname will match the newly created blog.)

• If $wpdb->insert_id matches by pure chance the correct blog_id: Task failed successfully.

[trepmal]

patch with test

[trepmal]

A plugin to reproduce the issue

[trepmal]

fix the ticket reference in the test

[SergeyBiryukov]

In 47932:

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.