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.
10 jobs for for_testrunner_1 in 0 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:15:31

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:17:01

failed SlapOS.SoftwareReleases.IntegrationTest-TestRunner1

07:27:21

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:42:45

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:16:01

passed SlapOS.Eggs.UnitTest-TestRunner1.Python2

00:27:19

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:18:24

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:33:15

passed SlapOS.Eggs.UnitTest-TestRunner1.Python3

00:16:45

failed SlapOS.SoftwareReleases.IntegrationTest-TestRunner1

07:33:49