Only look for WinSDK when building _for_ Windows

(Not when we're merely building _on_ Windows.)

CQ_INCLUDE_TRYBOTS=skia.primary:Build-Win-Clang-arm64-Release-Android

Change-Id: I7ab7663fcac6e3631ebe82f440927a077d476528
Reviewed-on: https://skia-review.googlesource.com/81483
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed