[py]: replace `platform.system()` with `sys.platform` (#16259)

Co-authored-by: Corey Goldberg <1113081+cgoldberg@users.noreply.github.com>
NOKEYCHECK=True
GitOrigin-RevId: c585e6a6e97801c0f16c6c84e2295c794b7f1e5f
6 files changed