Fix vpath build in libpq_pipeline test

The path needs to be set to refer to the build directory, not the
current directory, because that's actually the source directory at
that point.

fix for 6abc8c2596

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

@@ -8,14 +8,13 @@ use Config;
 use PostgresNode;
 use TestLib;
 use Test::More;
-use Cwd;
 
 my $node = get_new_node('main');
 $node->init;
 $node->start;
 
 my $numrows = 700;
-$ENV{PATH} = "$ENV{PATH}:" . getcwd();
+$ENV{PATH} = "$ENV{TESTDIR}:$ENV{PATH}";
 
 my ($out, $err) = run_command([ 'libpq_pipeline', 'tests' ]);
 die "oops: $err" unless $err eq '';