[cmake] Propagate DUMUX_MULTITHREADING_BACKEND downstream

The variable needs to be available. This used to be in the DumuxMacros which are executed again downstream. The modern way is to propagate via the config file we create for downstream modules finding dumux. Fix for !3980 (merged) which moved the variable to config but forgot to propagate it downstream. This means downstream modules didn't have enough information to compile against dumux.