Fix --req-ca to be used for build-ca

Default CN is already defined [here](https://github.com/jirutka/easy-rsa/blob/9038afcdf308372d512502d5fae96fad7980b8c6/easyrsa3/easyrsa#L1010). This export is undesirable and overwrites value passed with `--req-ca` or defined using `EASYRSA_REQ_CN` in vars.