openssl: Fix SSL_CERT_FILE to match ca-certs location

In OE-Core d6b15d1e,
"openssl: export necessary env vars in SDK", the value added for
SSL_CERT_FILE was in conflict with the value used elsewhere, such as
in buildtools. This makes them match and fixes buildtools testsdk
failures.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>