nixos-install: Pass CA cert bundle to chroot.

Since we're using HTTPS for the binary cache (introduced in faf0797c

) by
default, the binary cache should also be available during installation.

The file that is defined in SSL_CERT_FILE outside of the chroot is
copied over to /tmp/ca-cert.crt inside the chroot, so we have an
absolute path we can reference during nixos-install. However, this might
end up with the file not being cleaned up properly from outside of the
store, but neither would be /tmp/root so the cleanup issue needs to be
solved in another place (or commit to be more exact).

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