`env -u=2EKt` should generate an error

Found working on a fuzzer.
With our impl, it returns the env variable:

```
$  ./target/debug/coreutils env -u=2EKt  
ALACRITTY_LOG=/tmp/Alacritty-56177.log
[...]
```

With GNU:
```
$ LANG=C /usr/bin/env -u=2EKt
/usr/bin/env: cannot unset '=2EKt': Invalid argument
```