nix: fix ivory-header error conditions (detecting lfs pointers)
nix: fix ivory-header error conditions (detecting lfs pointers)

urbit/urbit,jfranklin9000/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,jfranklin9000/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit