cksum: unknown algorithm: blake3: clap should have prevented this case

```
$ cksum -a blake3 /dev/null
cksum: unknown algorithm: blake3: clap should have prevented this case
$ hashsum --b3sum /dev/null
af1349b9f5f9a1a6a0404dea36dcc9499bcb25c9adc112b7cc9a93cae41f3262  /dev/null
```