Theia: fix installation

This addresses both :
 - some random failures we sometimes have when installing `python-language-server` fail
 - caddy no longer being installed here since !821

See merge request !828
9 jobs for erp5-component
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:27:33

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

15:10:09

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

07:02:43

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

02:19:22

passed SlapOS.SlapRunner.ResilienceTest-Master

passed SlapOS.SlapRunner.ResilienceTest-Master

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

07:54:46

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3