erp5_web_service: add also prefix when writing temporary file
In practice, SFTP-based client processes often use a prefix based filter when retrieving files on the server.
ie: there is one SFTP server between 2 systems, used for different exchanges. The SFTP client will do something like (found in the wild): process1(file_list=sftp.list(path, prefix='PROCESS_1')) process2(file_list=sftp.list(path, prefix='PROCESS_2'))
Our implementation of temporary_file_on_write only sets a suffix, which doesn't work for the previous SFTP client. So let's add also a prefix when writing temporary files.