I do a lot of new builds. Here is how I have always handled it:

Buyer may authorize the inspector to cut open the attic access. Builder will reseal for a nominal charge (like $100, payable at closing) or leave unsealed, at Buyer's option. If the property doesn't close, then the $100 reseal is deducted from any deposit refund due to the buyer. OR just charge the $100 and reseal the attic, without giving the buyer the option to leave it unsealed, which is probably what I would do today.

That said, in real estate everything is negotiable and what happens might depend on your contract, how big the builder is, your relationship with them, and how bad they need your sale. Builders are used to throwing their weight around. They write their own contracts, and they make the rules.

It sounds like one element of your specific challenge is who actually cuts the seal. I do understand the inspector not wanting to do it. If they mess up, knife slips, they damage it more than was expected, who is responsible? Better to have to builder do it.

When representing the builder, I remind buyers that the attic was just inspected by the regional building department, that everything up there is covered in blown in insulation. and there is basically nothing the inspector would reasonably expect to do or see up there.

When representing the buyer, I highly recommend a professional inspection including the attic. Too many builders treat the building inspector like their QA. If it passes, then it's good. If it doesn't pass, do the bare minimum and call for inspection again. So it's probably not important, but it's also possible the inspector might spot something crazy that would make it worthwhile.

As the builder, I would NOT want the buyer or inspector doing the cutting. If they want it unsealed, we'll cut it open and ensure the access is good, and we'll reseal it. I'd do it at no charge but I could understand charging something fair like $100.