Temporarily using DOWNLOAD_CATCH=OFF for all Windows builds.

This also disables catch for some other builds that happen to be in the same
matrix. But there are still plenty of builds running with DOWNLOAD_CATCH=ON.

Workaround for a windows-only download issue that started yesterday (2020-06-23):

-- Downloading catch v2.13.2...
CMake Error at tools/FindCatch.cmake:40 (message):
  Could not download
  https://github.com/philsquared/Catch/releases/download/v2.13.2/catch.hpp
Call Stack (most recent call first):
  tools/FindCatch.cmake:59 (_download_catch)
  tests/pure_cpp/CMakeLists.txt:1 (find_package)