software/slapos-testing: use gcc 8.5 on python2

SlapOS.Eggs.UnitTest-Master.Python2 also installs old scipy so it needs
old gcc
9 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:09:41

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

01:33:41

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

09:26:14

passed SlapOS.Eggs.UnitTest-Master.Python2

01:11:58

passed SlapOS.Eggs.UnitTest-Master.Python3

00:22:06

passed SlapOS.Eggs.UnitTest-Master.Python3Next

00:14:17

failed SlapOS.SoftwareReleases.IntegrationTest-Master

07:21:56

passed SlapOS.Theia.ResilienceTest-Master-ERP5

06:28:20

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

00:46:09