psbt_sign_bip32: use the derived key to sign

[jgriffiths]

Fixes signing with parent keys, since the per-input signing call does not derive keys to sign with.

[JamieDriver]

utack c5e6243

This is similar to the [afaik working!] code in the Jade repo where the derived key is passed into wally_psbt_sign_input_bip32()