wallet: use correct derivation for elements when using mnemonic hsm_secret.

Even though we don't do taproot addresses on elements yet, use the
same scheme for simplicity and for future when we *do* do taproot.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

wallet/reservation.c

@@ -359,7 +359,6 @@ static struct command_result *finish_psbt(struct command *cmd,
 	/* Should we add a change output?  (Iff it can pay for itself!) */
 	change = change_amount(change, feerate_per_kw, weight);
 	if (amount_sat_greater(change, AMOUNT_SAT(0))) {
-		struct pubkey pubkey;
 		s64 keyidx;
 		u8 *b32script;
 		enum addrtype type;
@@ -378,8 +377,7 @@ static struct command_result *finish_psbt(struct command *cmd,
 					    " Keys exhausted.");
 
 		if (chainparams->is_elements) {
-			bip32_pubkey(cmd->ld, &pubkey, keyidx);
-			b32script = scriptpubkey_p2wpkh(tmpctx, &pubkey);
+			b32script = p2wpkh_for_keyidx(tmpctx, cmd->ld, keyidx);
 		} else {
 			b32script = p2tr_for_keyidx(tmpctx, cmd->ld, keyidx);
 		}
@@ -658,7 +656,6 @@ static struct command_result *json_addpsbtoutput(struct command *cmd,
 	u32 *locktime;
 	ssize_t outnum;
 	u32 weight;
-	struct pubkey pubkey;
 	s64 keyidx;
 	const u8 *b32script;
 	bool *add_initiator_serial_ids;
@@ -718,8 +715,7 @@ static struct command_result *json_addpsbtoutput(struct command *cmd,
 					    " Keys exhausted.");
 
 		if (chainparams->is_elements) {
-			bip32_pubkey(cmd->ld, &pubkey, keyidx);
-			b32script = scriptpubkey_p2wpkh(tmpctx, &pubkey);
+			b32script = p2wpkh_for_keyidx(tmpctx, cmd->ld, keyidx);
 		} else {
 			b32script = p2tr_for_keyidx(tmpctx, cmd->ld, keyidx);
 		}