fix(pumpfun): use is_cashback_coin instead of is_mayhem_mode for sell user_volume_accumulator

The Pump.fun IDL explicitly states that user_volume_accumulator is
passed as remaining_accounts[0] for cashback coins during sell.
Using is_mayhem_mode incorrectly omitted the account for cashback
tokens, causing the program to misread bonding_curve_v2 as
user_volume_accumulator and resulting in Custom(6024) Overflow.

src/instruction/pumpfun.rs

@@ -328,7 +328,7 @@ fn build_sell_v1(params: &SwapParams) -> Result<Vec<Instruction>> {
         accounts::FEE_PROGRAM_META,
     ];
 
-    if bonding_curve.is_mayhem_mode {
+    if bonding_curve.is_cashback_coin {
         metas.push(AccountMeta::new(user_volume_accumulator, false));
     }