I don't think anyone knows for sure at this point. We can guess they the base packs will probably be LFP. This guess is based on Rivian reporting that the base packs on R1 will be LFP in the future. As with anything else though its subject to change. LFP is desirable for base battery packs especially so it makes sense that Rivian would want to use LFP. The only issue is sourcing. It needs to qualify for the tax credit because those buyers are more likely to be cost sensitive.
No way to know at this stage, however LFP batteries keep ramping up energy density faster than Li-ion (I believe the latest CATL FePO4 batteries are now approaching 90% of energy density of Li-ion), so it would definitely make sense to keep prices as low as possible.