flake: Pin hyprland to pre-0.56.0 rev, keep own nixpkgs

Hyprland 0.56.0 dropped the hyprlang (.conf) config backend entirely
in favor of a Lua-only config, breaking the deployed hyprland.conf on
pinwheel. Pin back to 79bc0ca (last hyprlang-supporting build) until
the config is migrated to Lua.

Also drop hyprland's nixpkgs.follows override: building the old
Hyprland rev against the current (newer) root nixpkgs caused a
libstdc++ ABI mismatch between the cached hyprtoolkit binary and a
freshly-compiled hyprland-guiutils.

flake.nix

@@ -33,8 +33,9 @@
     };
 
     hyprland = {
-      url = "github:hyprwm/Hyprland";
-      inputs.nixpkgs.follows = "nixpkgs";
+      # Pinned pre-0.56.0: 0.56.0 dropped the hyprlang (.conf) config
+      # backend in favor of Lua-only config. Bump once migrated.
+      url = "github:hyprwm/Hyprland/79bc0ca16e7cca40c247a9200634d150fa8193d1";
     };
 
     hyprland-contrib = {