software/slapos-sr-testing: correctly disallow usage of system python3

previous patch - 24f5a96c (software/slapos-sr-testing: disallow usage
of system python3, 2023-10-02) - was incorrect, an `entry-point` is
required for a script to be generated.
3 jobs for for_testrunner_1 in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:30:42

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:24:04

failed SlapOS.SoftwareReleases.IntegrationTest-TestRunner1

06:06:39