nix: fix overlay composition

[GrizzlT]

Describe your PR, what does it fix/add?

The nix overlay defined in nix/overlays.nix defines a function called mkJoinedOverlays to combine the correct dependencies for hyprland. This function however only combines the result of multiple overlays but does not provide the dependency overlay outputs to callPackage as an input when defining the hyprland package. This means that the internal dependency wlroots-hyprland must be defined on the final nixpkgs fixpoint.

To avoid name clashes, I personally shadow my flake input's overlays to provide a cleaner package set when selecting packages (i.e. hide internal dependencies). Because the hyprland overlay depends on the final package set to have a root wlroots-hyprland package, this prohibits my workflow. This PR fixes that.

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

This fix should be fully backwards compatible. I have tested that the overlay output package is a derivation. I have not tested the theoretical possibility of still manually overriding wlroots-hyprland using hyprland.override { ... }. This should however work since the hyprland package is a standard mkDerivation result.

Is it ready for merging, or does it need work?

ready, do I need formatting?

[fufexan]

LGTM. Thanks!