Since we're using HTTPS for the binary cache (introduced in faf0797) 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>