Sunshine assumes assets directory is relative to current directory for box art images

[ReenigneArcher]

https://github.com/SunshineStream/Sunshine/blob/c75efd399960422f2c3c69b930f30122f147c6f1/sunshine/process.cpp#L214-L217

Something like this might work.

  std::error_code code;
  if(!std::filesystem::exists(app_image_path, code) || image_extension != ".png") {
    if(!std::filesystem::exists(SUNSHINE_ASSETS_DIR "/" app_image_path, code)) {
        return SUNSHINE_ASSETS_DIR "/box.png";
    }
  }

Update apps.json files as well:
https://github.com/SunshineStream/Sunshine/blob/e7ec6050d9a820549d556bf80e6dcb17460ee996/src_assets/linux/config/apps.json#L17
https://github.com/SunshineStream/Sunshine/blob/e7ec6050d9a820549d556bf80e6dcb17460ee996/src_assets/windows/config/apps.json#L11
"image-path":"steam.png"

[github-actions]

This issue has been fixed and will be available in the next release.