buildtools-tarball: export OPENSSL_CONF in environment setup

The autobuilder has been experiencing SSL: CERTIFICATE_VERIFY_FAILED
errors during error report uploads when using buildtools due to looking
for certs in /opt/poky

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>