flake: update for swayfx 0.4

[ozwaldorf]

Summary

• Use new scenefx flake+overlay
• Populate the scenefx subproject location with the source in devshell like we do for wlroots
• Document the override overlay requirement for scenefx
• Build swayfx 0.4!

Required PRs

• https://github.com/wlrfx/scenefx/pull/37

• 290

Notes

Once scenefx has a tagged release, we can target that for the flake input. For now, the repo will need a nix flake update to use newer iterations of scenefx. Keep in mind this will need to be maintained as scene releases new versions, of course.

I'm also not entirely sure if the override overlay makes sense, I went with a generic pattern where users are required to overlay their own scenefx into nixpkgs (or in the future, nixpkgs will have scenefx natively and that would be used)

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Passing Garnix CI

[WillPower3309]

Only comment is that we can update the scene input now that the flake is merged there, otherwise LGTM!