Tests: Look for Visual Studio only on Windows hosts

Update logic added by commit 6051a49c (Visual Studio: Add Android
support, 2020-06-12, v3.19.0-rc1~619^2) to only look for VS
installations using `cmake_host_system_information` on Windows hosts.
Otherwise the `VS_*_DIR` keys are not available.

Fixes: #21547
13 jobs for test-vs-windows-hosts