wip: Fix flake

[keysmashes]

flake.nix has bitrotted and doesn't currently work (for example: Alpha has moved to .NET 9, but the flake still references .NET 8). These are some of the changes required to get it working.

Currently this isn't sufficient to get it running; I need to specify a LD_LIBRARY_PATH that contains libSDL2-2.0.so (symlink to ${SDL2}/lib/libSDL2-2.0.so.0.3000.6), since that file is not available under that name in NixOS anywhere, as far as I can tell, and I don't remember what the correct way to fix this in the flake is (probably patchelf?). Also, I get frequent assertion failures in imgui; for example, when opening the "Filesystem" window:

Alpha: /home/runner/work/Hexa.NET.ImGui/Hexa.NET.ImGui/cimgui/imgui/imgui.cpp:11065: void ImGui::ErrorCheckUsingSetCursorPosToExtendParentBoundaries(): Assertion `0 && "Code uses SetCursorPos()/SetCursorScreenPos() to extend window/parent boundaries. Please submit an item e.g. Dummy() to validate extent."' failed.

and when interacting with the "Excel" window:

Alpha: /home/runner/work/Hexa.NET.ImGui/Hexa.NET.ImGui/cimgui/imgui/imgui.cpp:3096: void ImGuiListClipper::End(): Assertion `data->ListClipper == this' failed.

which are apparently not present when running on windows.