Fix bug limit_num_jobs=0

3 jobs for topic/default/0.4.7.post0 in 11 minutes
Status Job ID Name Coverage
  Test
passed #162652
step_pythran_cython

00:10:01

passed #162651
step_pythran_then_cython

00:11:00

passed #162650
step_without_pythran

00:04:02