pgsql: Skip psql's TAP test for query cancellation entirely on Windows

Skip psql's TAP test for query cancellation entirely on Windows

This changes 020_cancel.pl so as the test is entirely skipped on
Windows.  This test was already doing nothing under WIN32, except
initializing and starting a node without using it so this shaves a few
test cycles.

Author: Yugo NAGATA
Reviewed-by: Fabien Coelho
Discussion: https://postgr.es/m/20230810125935.22c2922ea5250ba79358965b@sraoss.co.jp
Backpatch-through: 15

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/3f7b7d9df2dc7aa14add615c7af8fbc1092509b9

Modified Files
--------------
src/bin/psql/t/020_cancel.pl | 118 +++++++++++++++++++++----------------------
1 file changed, 59 insertions(+), 59 deletions(-)