pgsql: Fix failures in incremental_sort due to number of workers

Fix failures in incremental_sort due to number of workers

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/23ba3b5ee278847e4fad913b80950edb2838fd35

Modified Files
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src/test/regress/expected/incremental_sort.out | 1 +
src/test/regress/sql/incremental_sort.sql      | 1 +
2 files changed, 2 insertions(+)