Switch VS2017 compile bots to Windows 10.

Per tandrii, these may work better.

Change-Id: I2719035d6f2a06b2baeb7fe820d77e64d0e9542b
Reviewed-on: https://boringssl-review.googlesource.com/29164
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: David Benjamin <davidben@google.com>