[pdfx] failing to build on Windows

[ekuleshov]

Describe the bug
Getting the following error when building a Windows desktop app on Apple M1 under Parallels running Windows arm64.

This build works with other packages based on PDFium, e.g. printing.

Additional context

CMake Error at C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.20/Modules/ExternalProject.cmake:2771 (message):
  At least one entry of URL is a path (invalid in a list)
Call Stack (most recent call first):
  C:/Program Files (x86)/Microsoft Visual Studio/2019/BuildTools/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.20/Modules/ExternalProject.cmake:3681 (_ep_add_download_command)
  CMakeLists.txt:9 (ExternalProject_Add)


CMake Error at flutter/ephemeral/.plugin_symlinks/pdfx/windows/DownloadProject.cmake:171 (message):
  CMake step for pdfium failed: 1
Call Stack (most recent call first):
  flutter/ephemeral/.plugin_symlinks/pdfx/windows/CMakeLists.txt:20 (download_project)


Error: Unable to generate build files