erp5_web_service: add also prefix when writing temporary file

See merge request !1481
5 jobs for listbox_use_domsugar in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.UnitTest-Master

01:43:35

passed SlapOS.Eggs.UnitTest-Master.Python2

00:08:14

passed SlapOS.Eggs.UnitTest-Master.Python3

00:08:19

passed SlapOS.Eggs.UnitTest-Master.Python2

00:08:13

passed SlapOS.Eggs.UnitTest-Master.Python3

00:38:40