Modify fn signature for add_witness_to_circuit_description

@kevaundray was right and there's a method that does the exact same thing called: `add_witness_to_circuit_description` but it asks fora `Variable` and a `Scalar` when it could just ask for a `Scalar` and return the `Variable`. So I'll create an issue in order to address that.

_Originally posted by @CPerezz in https://github.com/dusk-network/plonk/issues/280#issuecomment-673429897_