`env -0 -u=o 6=C` should be rejected

With our impl:
```
$ ./target/debug/coreutils env -0 -u=o 6=C
LACRITTY_LOG=/tmp/Alacritty-56177.logALACRITTY_SOCKET=/run/user[...]

```

With GNU:
```
$ LANG=C /usr/bin/env -0 -u=o 6=C
/usr/bin/env: cannot unset '=o': Invalid argument
```