nix: overlays: remove mkJoinedOverlays function

Closes #285

I introuced `mkJoinedOverlays` long ago, but we can use
`lib.composeManyExtensions` instead. I am happy to remove it!

nix/overlays.nix

@@ -5,9 +5,6 @@
 }: let
   ver = lib.removeSuffix "\n" (builtins.readFile ../VERSION);
 
-  mkJoinedOverlays = overlays: final: prev:
-    lib.foldl' (attrs: overlay: attrs // (overlay final prev)) {} overlays;
-
   mkDate = longDate: (lib.concatStringsSep "-" [
     (builtins.substring 0 4 longDate)
     (builtins.substring 4 2 longDate)
@@ -16,7 +13,7 @@
 
   version = ver + "+date=" + (mkDate (self.lastModifiedDate or "19700101")) + "_" + (self.shortRev or "dirty");
 in {
-  default = mkJoinedOverlays (with self.overlays; [
+  default = lib.composeManyExtensions (with self.overlays; [
     xdg-desktop-portal-hyprland
     inputs.hyprlang.overlays.default
     inputs.hyprland-protocols.overlays.default