The last test in incremental_sort suite prints a parallel plan, but some
of the buildfarm animals have custom max_parallel_workers_per_gather
values, causing failures. Fixed by setting the GUC to an explicit value.

Discussion: https://postgr.es/m/CAPpHfds1waRZ=NOmueYq0sx1ZSCnt+5QJvizT8ndT2=etZEeAQ@mail.gmail.com