playbook: actually Ubuntu 20.04 doesn't support https URL

fixup of b68b3f17
4 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

10:11:19

passed ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

00:32:05

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

09:40:24

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

00:28:12