`cksum -a blake2b .` should generate an error

Currently:
```
$ ./target/debug/coreutils cksum -a blake2b .
BLAKE2b (.) = 786a02f742015903c6c6fd852552d272912f4740e15847618a86e217f71f5419d25e1031afee585313896444934eb04b903a685b1448b755d56f701afe9be2ce
```

GNU:
```
$ /usr/bin/cksum -a blake2b .
/usr/bin/cksum: .: Is a directory
```

Tested by tests/misc/read-errors.sh