Disable implicit shardCollection after drop on OS X in implicitly_shard_accessed_collections.js

The changes from 56e6645 as part of SERVER-26315 have caused Evergreen tasks to time out when running the sharded_collections_jscore_passthrough and sharded_collections_jscore_passthrough_WT tasks on the OS X builders. We should disable the implicit call to the "shardCollection" command until SERVER-28418 is completed.

This can be done with a check along the lines of db.serverBuildInfo().buildEnvironment.target_os === "macOS".