nixpkgs: fix chez build failures

The Chez build was failing, as usual, due to impurities. The build
system refers to absolute paths for tools like `ln` or `true`, which
was the real culprit here. Furthermore the build also 'helpfully'
suppresses errors in these cases by piping to /dev/null, so you never
see any errors at build time until it's too late (otherwise, you'd
see failures to call /bin/ln or at ./configure time).

This also re-enables parallel builds, as they should be safe from
all my testing, I believe.

Signed-off-by: Austin Seipp <aseipp@pobox.com>