plugins: imapsieve: Fix assert panic occurring when a COPY event is triggered on a virtual mailbox.

Occurs when several mails from different backend mailboxes are involved in the
COPY event. Fixed by using mail_save_context->>copy_src_mail in mailbox_copy()
instead of the mail argument. The latter can point to the backend mailbox for
virtual mailboxes, which makes no sense to the imapsieve plugin as it expects
only one source mailbox.

Panic was:

Panic: file imap-sieve-storage.c: line 337 (imap_sieve_add_mailbox_copy_event): assertion failed: (ismt->src_box == NULL || ismt->src_box == src_mail->box)