Make sure that your daemon nix config at: /etc/nix/nix.conf (or
/etc/nix/nix.custom.conf) contains:
extra-trusted-user = <user-name>to enable substituters in flake.nix.
Updating works with cd tools/nix && nix flake update multiverse and then
adjusting the pinned nixpkgs sets in
nixpkgs.parts.nix. Do a direnv reload
and fix the asserts.