Networks and Sites: Fix incorrect variable location.

This fixes an issue introduced in [44166] where the `$groupby` variable was inserted too low in the `get_site_ids()` function while merging [43832] into `trunk`. The merged location did not account for a new conditional statement that existed only in `trunk`, and would have resulted in values assigned to `$groupby` being erased in certain scenarios.

Props spacedmonkey.

See #44416.
Fixes #45582.
Built from https://develop.svn.wordpress.org/trunk@44186


git-svn-id: https://core.svn.wordpress.org/trunk@44016 1a063a9b-81f0-0310-95a4-ce76da25c4cd