nixos/nix-optimise: run with lower priority (#420721)

    systemd = lib.mkIf config.nix.enable {

      services.nix-optimise = {

        description = "Nix Store Optimiser";

        unitConfig = {

          ConditionACPower = true;

          # No point this if the nix daemon (and thus the nix store) is outside

        unitConfig.ConditionPathIsReadWrite = "/nix/var/nix/daemon-socket";

        serviceConfig.ExecStart = "${config.nix.package}/bin/nix-store --optimise";

          ConditionPathIsReadWrite = "/nix/var/nix/daemon-socket";

        };

        serviceConfig = {

          ExecStart = "${lib.getExe' config.nix.package "nix-store"} --optimise";

          Nice = 19;

          CPUSchedulingPolicy = "idle";

          IOSchedulingClass = "idle";

        };

        startAt = lib.optionals cfg.automatic cfg.dates;

        # do not start and delay when switching

        restartIfChanged = false;