nixos/testing: Don't shadow "lib" in makePerlPath.

Regression introduced by 032f0ffd

.

The change doesn't look obvious at the first sight why it may cause
problems with lib.makePerlPath, but it introduces a Perl package called
"lib".

And using "with perlPackages;" uses the Perl library "lib" instead of
the lib attribute set from pkgs.

So let's use pkgs.lib.makePerlPath directly in hope that there won't be
a Perl package anytime soon which is called "pkgs".

Signed-off-by: aszlig <aszlig@redmoonstudios.org>