Define THOR_BUILD_TEST_FUNCTOR_CACHE cache variable by default for upgrade slots
lhcb/Rec!1742 (merged) added the possibility of building a functor cache from one of the test scripts.
This is enabled by the THOR_BUILD_TEST_FUNCTOR_CACHE
variable. We want this to be set in the nightlies, but not for all users.
cc: @rmatev