nixos/cron: Fix ShellCheck issues (#370910)

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

        preStart = ''

          mkdir -m 710 -p /var/cron

          (umask 022 && mkdir -p /var)

          (umask 067 && mkdir -p /var/cron)

          # By default, allow all users to create a crontab.  This

          # is denoted by the existence of an empty cron.deny file.