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: https://postgr.es/m/20210930214040.odkdd42vknvzifm6@alap3.anarazel.de
Backpatch: 14-, where the test was introduced.

src/test/modules/libpq_pipeline/t/001_libpq_pipeline.pl

@@ -14,9 +14,9 @@ $node->init;
 $node->start;
 
 my $numrows = 700;
-$ENV{PATH} = "$ENV{TESTDIR}:$ENV{PATH}";
+my $libpq_pipeline = "$ENV{TESTDIR}/libpq_pipeline";
 
-my ($out, $err) = run_command([ 'libpq_pipeline', 'tests' ]);
+my ($out, $err) = run_command([ $libpq_pipeline, 'tests' ]);
 die "oops: $err" unless $err eq '';
 my @tests = split(/\s+/, $out);
 
@@ -39,7 +39,7 @@ for my $testname (@tests)
 	# Execute the test
 	$node->command_ok(
 		[
-			'libpq_pipeline', @extraargs,
+			$libpq_pipeline, @extraargs,
 			$testname,       $node->connstr('postgres')
 		],
 		"libpq_pipeline $testname");