buildEnv: Pass the right number of args to addPkg

Regression introduced by 4529ed12590d7f814e40223437c7df4fea492e2d.

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 <[e-mail]>
Fixes: #12635

pkgs/build-support/buildenv/builder.pl

···
     my @pkgDirs = keys %postponed;
     %postponed = ();
     foreach my $pkgDir (sort @pkgDirs) {
-        addPkg($pkgDir, 2, $priorityCounter++);
+        addPkg($pkgDir, 2, $ENV{"checkCollisionContents"} eq "1", $priorityCounter++);
     }
 }