Add mac platform to shard utils.

This is required for the platform to recipe name translations.

Change-Id: Iee8d02011ecc75bafe58bc9a17359f83c9f567bf
Bug: https://github.com/flutter/flutter/issues/65223
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6322
Reviewed-by: Tong Wu <wutong@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>