GitHub Actions: Windows builds fail to download GLFW3 with vcpkg

[ASchuberth]

In Run CMake step in build.yml, vcpkg fails to download GLFW3.

See log of a previous build: log

      CMake Error at scripts/cmake/vcpkg_download_distfile.cmake:32 (message):
      
      Failed to download file with error: 1
      If you are using a proxy, please check your proxy setting. Possible causes are:
      
      1. You are actually using an HTTP proxy, but setting HTTPS_PROXY variable
         to `https://address:port`. This is not correct, because `https://` prefix
         claims the proxy is an HTTPS proxy, while your proxy (v2ray, shadowsocksr
         , etc..) is an HTTP proxy. Try setting `http://address:port` to both
         HTTP_PROXY and HTTPS_PROXY instead.
      
      2. If you are using Windows, vcpkg will automatically use your Windows IE Proxy Settings
         set by your proxy software. See https://github.com/microsoft/vcpkg-tool/pull/77
         The value set by your proxy might be wrong, or have same `https://` prefix issue.
      
      3. Your proxy's remote server is out of service.
      
      If you've tried directly download the link, and believe this is not a temporary
      download server failure, please submit an issue at https://github.com/Microsoft/vcpkg/issues
      to report this upstream download server failure.```