fix(CI): Use the proper image for tox command

5 jobs for branch/default in 3 minutes and 2 seconds (queued for 4 seconds)
Name Stage Failure
failed
py3 Tests
ERROR test/unittest_site_cubicweb.py
ERROR test/unittest_sobjects_skos.py
!!!!!!!!!!!!!!!!!!! Interrupted: 19 errors during collection !!!!!!!!!!!!!!!!!!!
======================== 3 warnings, 19 errors in 6.29s ========================
ERROR: InvocationError for command '/builds/cubicweb/cubes/saem_ref/.tox/py3/bin/python -m pytest /builds/cubicweb/cubes/saem_ref/test' (exited with code 2)
___________________________________ summary ____________________________________
ERROR: py3: commands failed
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1