pgsql: Reference test binary using TESTDIR in 001_libpq_pipeline.pl.

Reference test binary using TESTDIR in 001_libpq_pipeline.pl.

The previous approach didn't really work on windows, due to the PATH separator
being ';' not ':'. Instead of making the PATH change more complicated,
reference the binary using the TESTDIR environment.

Reported-By: Andres Freund <andres@anarazel.de>
Suggested-By: Andrew Dunstan <andrew@dunslane.net>
Discussion: /messages/by-id/20210930214040.odkdd42vknvzifm6@alap3.anarazel.de
Backpatch: 14-, where the test was introduced.

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0baa33da0fd123aff215c505116dc60bc04dae90

Modified Files
--------------
src/test/modules/libpq_pipeline/t/001_libpq_pipeline.pl | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)