Circuit: Check pk_d_old = derived_pk_d_old only when split_flag = 0

[ConstanceBeguier]

In the circuit derived_pk_d_old is evaluated from rivk, ak, nk and g_d_old.
rivk, ak and nk comes from the FullViewingKey stored in the spent note.
For split note, the FullViewingKey stored in the spent note is random in order to derive a random Nullifier nf_old.
Thus, the constraint pk_d_old = derived_pk_d_old must not be checked for split note (split_flag=1).

[what-the-diff]

PR Summary

• Add a new constraint to the Orchard circuit checks
  This change introduces an additional constraint to ensure the split_flag is set to 1 or the derived public key remains the same as the original public key. This helps in maintaining data consistency and correct functioning of the system.