vnstat: 1.17 -> 1.18

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstat --help` got 0 exit code
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstat -v` and found version 1.18
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstat --version` and found version 1.18
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstat --help` and found version 1.18
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstati --help` got 0 exit code
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstati -v` and found version 1.18
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstati --version` and found version 1.18
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstati --help` and found version 1.18
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstatd --help` got 0 exit code
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstatd -v` and found version 1.18
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstatd --version` and found version 1.18
- ran `/nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18/bin/vnstatd --help` and found version 1.18
- found 1.18 with grep in /nix/store/zaidqa5gzm2wdfg0mrybj180k73jbkpq-vnstat-1.18