CI: Run batch scripts on windows with cmd, so environment variables get expanded

[jayrulez]

This should fix the vulkan issues in the build pipelines.

[jayrulez]

GitHub’s Windows runners define VULKAN_SDK as an empty environment variable. The old check: if ("%VULKAN_SDK%"=="") fails because an empty defined variable does not satisfy the string check, causing the script to enter the "SDK found" branch with an empty value.

Replaced the condition with: if not defined VULKAN_SDK

[jayrulez]

The release build is failing with a linker error. I don't get it locally, using my installed SDK. Maybe there is a problem with the vulkan sdk linked in 3rdp packages.

[jayrulez]

Oh, and it seems the pipeline doesn't detect the build failure when powershell is used. Maybe it's better to go back to specifying cmd as the shell to run the .bat files.