Allow SBE to be used on the inner side of a DocumentSourceLookup

SERVER-69103 proposes a change to always use the classic engine for queries issued recursively by DocumentSourceLookup. In the long run, as SBE becomes more mature and more performant, we may want to re-enable the ability for it to be used on the inner side of a DocumentSourceLookup. Note that this only applies to $lookup operations which execute inside the DocumentSource engine, not to those which are pushed down entirely to SBE or potentially those which are handled by the Bonsai optimizer in the future.