nixos/scx: support temporarily overriding scheduler (#453196)

      serviceConfig = {

        Type = "simple";

        ExecStart = utils.escapeSystemdExecArgs (

          [

            (lib.getExe' cfg.package cfg.scheduler)

          ]

          ++ cfg.extraArgs

        );

        ExecStart = ''

          ${pkgs.runtimeShell} -c 'exec ${cfg.package}/bin/''${SCX_SCHEDULER_OVERRIDE:-$SCX_SCHEDULER} ''${SCX_FLAGS_OVERRIDE:-$SCX_FLAGS}'

        '';

        Restart = "on-failure";

      };

      environment = {

        SCX_SCHEDULER = cfg.scheduler;

        SCX_FLAGS = lib.escapeShellArgs cfg.extraArgs;

      };

      wantedBy = [ "multi-user.target" ];

    };