https://wiki.hypr.land/Nix/Cachix/
I've gotten this warning ignoring the client-specified setting 'trusted-public-keys', because it is a restricted setting and you are not a trusted user and fixed it by adding trusted-users:
nix.settings = {
substituters = [ "https://hyprland.cachix.org" ];
trusted-substituters = [ "https://hyprland.cachix.org" ];
trusted-public-keys = [ "hyprland.cachix.org-1:a7pgxzMz7+chwVL3/pzj6jIBMioiJM7ypFP8PwtkuGc=" ];
trusted-users = [ # Enter either @wheel for users with sudo permission, or your username
"@wheel"
"yourusername"
];
};