I `nix-channel --add`ed 20.03 over 19.03 as root, then `nix-channel --update`d, and finally `nixos-rebuild switch`. After downloading over a gigabyte of stuff, it finally got to the GRUB part:
`updating GRUB 2 menu...` / `installing the GRUB 2 boot loader on …...` / `Installing for i386-pc platform` /…
`…/grub-install: warning: your core.img is unusually large. It won't fit in the embedding area.`
`…/grub-install: error: filesystem 'btrfs' doesn't support blocklists.`
`…-install-grub.pl: installation of GRUB on … failed`
`warning: error(s) occurred while switching to the new configuration`