Update the "os" dimension of Windows tasks to Windows-19041.

This is a cherry-pick of https://skia-review.googlesource.com/c/skia/+/418316/.

Change-Id: I70895609847f6f987ccdaa0e6af7c71c13f821fe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/418816
Commit-Queue: Leandro Lovisolo <lovisolo@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Auto-Submit: Leandro Lovisolo <lovisolo@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>