obs/slapos: fix use shorter name

fixup for 203927d9

The old tarball was not removed correctly
8 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

15:59:53

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:53:17

failed ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

16:34:06

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

15:56:37

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

16:40:25

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

16:27:12

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

16:37:32

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch

15:54:57