{ nixpkgs.config.allowUnsupportedSystem = true; }
       in configuration.nix to override this.

       c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
         { allowUnsupportedSystem = true; }
       to ~/.config/nixpkgs/config.nix.

Cannot nix-instantiate `exercise-timer.passthru.tests` because:
error: attribute 'tests' in selection path 'exercise-timer.passthru.tests' not found