Windows workflows suddenly fail with include path for SDL not being found

Github workflow is suddenly having trouble finding include paths for the SDL dependency.

Last good run: https:// github .com/SuperTux/supertux/actions/runs/1205492779
First bad run: https:// github .com/SuperTux/supertux/actions/runs/1214704128

Last good commit: https:// github .com/SuperTux/supertux/commit/63a326685f72ffb66e241c5432a07a8737931f4e
First bad commit: https:// github .com/SuperTux/supertux/commit/622bef9751b39a7cc34782b7f762070d5d2fd0cb

Logs are here:
https:// github .com/SuperTux/supertux/runs/3548835223?check_suite_focus=true

I have no idea why the SDL include would suddenly start failing. Our CMakeLists.txt is here if anyone wants to take a look at that:
https:// github .com/SuperTux/supertux/blob/master/CMakeLists.txt