Let's not have a package named "lib"

See fb24c7c95b.

The "_" suffix follows the convention of the "if" package.

nixos/lib/testing.nix

@@ -28,7 +28,7 @@ rec {
 
         wrapProgram $out/bin/nixos-test-driver \
           --prefix PATH : "${qemu_kvm}/bin:${vde2}/bin:${netpbm}/bin:${coreutils}/bin" \
-          --prefix PERL5LIB : "${with perlPackages; pkgs.lib.makePerlPath [ TermReadLineGnu XMLWriter IOTty FileSlurp ]}:$out/lib/perl5/site_perl"
+          --prefix PERL5LIB : "${with perlPackages; lib.makePerlPath [ TermReadLineGnu XMLWriter IOTty FileSlurp ]}:$out/lib/perl5/site_perl"
       '';
   };