Remove incorrect cloning of InternalVar in acir_gen

[kevaundray]

Problem

In acir_gen, each time we fetch an InternalVar using a NodeId and generate a cached_witness that cached_witness is not saved back into the internal_var_cache, so fetching the same InternalVar again using the same NodeId will not show the cached_witness.

Solution

• One solution is to update the InternalVarCache whenever we generate a witness for an expression in an InternalVar

Alternatives considered

Additional context