buildEnv: Pass the right number of args to addPkg

Regression introduced by 4529ed12

.

I've missed this in #5096, not because of a messed up rebase as I have
guessed from a comment on #12635 but missed this in the first place.

The testing I did while working on the pull request weren't exhaustive
enough to cover this, because I haven't tested with packages that use
the propagatedUserEnvPkgs attribute.

In order to make the test a bit more exhaustive this time, let's test it
using:

nix-build -E 'with import ./. {}; buildEnv {
  name = "testenv";
  paths = [
    pkgs.hello pkgs.binutils pkgs.libsoup pkgs.gnome3.yelp
    pkgs.gnome3.totem
  ];
}'

And with this commit the errors no longer show up and the environment is
built correctly.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Fixes: #12635