PATH manipulation in 001_libpq_pipeline.pl fails on windows

Hi,

For me 001_libpq_pipeline.pl doesn't reliably work on windows, because it
tries to add something to PATH, using unix syntax (vs ; used on windows).

$ENV{PATH} = "$ENV{TESTDIR}:$ENV{PATH}";

If the first two elements in PATH are something needed, this can cause the
test to fail... I'm surprised this doesn't cause problems on the buildfarm - a
plain
  perl src\tools\msvc\vcregress.pl taptest src\test\modules\libpq_pipeline\
fails for me.

Greetings,

Andres Freund