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>
(cherry picked from commit 7d383a7f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
(cherry picked from commit a9a50f2216951e26b62ed2f86f341d9ad13acf48)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>