fix(svm): N-04 reset use of PDA signer in multicall handler

[Reinis-FRP]

OZ identified following issue:

Across deposits may include additional message data that must be processed during relayer fills, in which
case the data is deserialized and processed as various accounts and instructions that are invoked on the
message handler specified in the relayer data. In the provided multicall_handler example, the handler_signer
may be included as an additional signer in the CPI calls decoded from the message. However, it may be
included when not necessary. This is because, in each program call, use_handler_signer is set if any accounts
in the call match the handler_signer key, but it is never reset to false before checking subsequent calls.

Consider correcting this logic by resetting the value of use_handler_signer at the top of the outermost for
loop to avoid passing additional unnecessary signers in CPI calls.

This PR addresses the issue by resetting use_handler_signer to false at the start of processing each instruction.

Fixes: https://linear.app/uma/issue/ACX-3593/n-04-unneccessary-pda-signer-can-be-used-in-multicall-handler

[linear]

ACX-3593 N-04 Unneccessary PDA Signer Can Be Used in Multicall Handler