fix "slapos/obs: Force gcc from slapos"

Setting max_version to 0 is less hacky. Also, it's not only for tarball
preparation, the package is created the same way.
7 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:33:06

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:08:31

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

05:30:09

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

02:04:51

passed SlapOS.SlapRunner.ResilienceTest-Master

00:25:58

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:06:28

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:08:18