download: only take basename of 'filename' when the latter is used to calculate destination path

Currently only useful for buildout.hash.cfg files in
https://lab.nexedi.com/nexedi/slapos (e.g. software/slaprunner)
6 jobs for master
Status Job ID Name Coverage
  External
running SlapOS.Eggs.UnitTest-Master.Python2

21126:26:55

passed SlapOS.Eggs.UnitTest-Master.Python3

00:27:12

passed SlapOS.Eggs.UnitTest-Master.Python2

00:50:42

passed SlapOS.Eggs.UnitTest-Master.Python2

00:22:29

passed SlapOS.Eggs.UnitTest-Master.Python3

03:19:34

passed SlapOS.Eggs.UnitTest-Master.Python3

00:35:03