OSX SSL builds should use unique filename

To match Windows, which is the other platform with both platform-specific non-SSL and SSL Community builds.

For example from the logs you can see both of these tasks pushed to http://downloads.mongodb.org/osx/mongodb-osx-x86_64-latest.tgz -
https://evergreen.mongodb.com/task/mongodb_mongo_master_osx_108_push_5bf56982d557a6c4e7d156b1e525950dd7960684_15_06_06_09_51_59/0?type=ALL
https://evergreen.mongodb.com/task/mongodb_mongo_master_osx_108_ssl_push_4ea38c308da292f43e29d32b1b53b7324db0bafe_15_05_29_14_28_37#/log/ALL