fix: override the original witnesses when sign transactions

[yangby-cryptape]

Issue

I'm working on a new type script, which requires its own witness.

But when I build a transaction with ckb_sdk::XxxUnlocker, the witness will be overridden.

• When sign transactions, the method to handle witnesses are correct.

  ckb-sdk-rust/src/unlock/signer.rs

  Lines 109 to 119 in c45c4dc

  	// Put signature into witness
  	let witness_data = witnesses[witness_idx].raw_data();
  	let mut current_witness: WitnessArgs = if witness_data.is_empty() {
  	WitnessArgs::default()
  	} else {
  	WitnessArgs::from_slice(witness_data.as_ref())?
  	};
  	current_witness = current_witness
  	.as_builder()
  	.lock(Some(signature).pack())
  	.build();

• But, when call the following code to prepare for sign transactions:

  ckb-sdk-rust/src/transaction/builder/mod.rs

  Line 182 in c45c4dc

  if handler.build_transaction(&mut tx, script_group, context.as_ref())? {

  The original witnesses will be overridden.

I tested this patch with my project, it works!