All search bars in Nova Launcher use Google. Can I change this?
Firstly, remove the Google Search bar. After that, hold anywhere in the homescreen where there ins't any icons. Click "Widgets". Then browse to the widget of the search bar of your choice.
Along with the other answer, there is another way to change Nova Launcher's default search engine to use (although it is not very ideal unless you want to keep the Nova search widget).
To do it, disable all your search apps besides the one you want to use, and try to use the Search bar again.
It should give you the search app that you want to use.
I used it to switch from my default Search app (
com.android.quicksearchbox) to Google Search.