lei_mail_sync: explicit bind for old SQL_VARCHAR compat
This avoids repeated work for incremental "lei import" runs when
users upgrade from 1.7 to current public-inbox.git (and eventually
1.8).
We need the explicit bind_param for fallback calls because
previous bind_param calls are "sticky" for a given statement
handle. The DBI(3pm) manpage states:
The data type is 'sticky' in that bind values passed to execute()
are bound with the data type specified by earlier bind_param()
calls, if any. Portable applications should not rely on being
able to change the data type after the first "bind_param" call.