Update the version of cpython that test shards grab from CIPD.

The current version does not have a package for Windows ARM64. Update to the version with the "latest" label, which does have a Windows ARM64 package.

Change-Id: I65d53271a1b9a77edce9feda57b4eb5792d269b7
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/504322
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>