Generate overlay with final and prev parameters

Otherwise `nix flake check` complains with:

    error: overlay does not take an argument named 'final'

This also changes which callPackage is called. Previously, we used
"super" (now prev), but we changed it to "final", because this seems
to be more correct (i.e. used in nixpkgs).

ros2nix/ros2nix.py

@@ -100,14 +100,14 @@ def file_writer(path: str, compare: bool):
 
 def generate_overlay(expressions: dict[str, str], args):
     with file_writer(f'{args.output_dir or "."}/overlay.nix', args.compare) as f:
-        print("self: super:\n{", file=f)
+        print("final: prev:\n{", file=f)
         for pkg in sorted(expressions):
             expr = (
                 expressions[pkg]
                 if args.output_dir is None
                 else f"./{os.path.basename(expressions[pkg])}"
             )
-            print(f"  {pkg} = super.callPackage {expr} {{}};", file=f)
+            print(f"  {pkg} = final.callPackage {expr} {{}};", file=f)
         print("}", file=f)
 
 
@@ -119,9 +119,9 @@ ros_distro_overlays_def = dedent(
       // builtins.mapAttrs (
         rosDistro: rosPkgs: if rosPkgs ? overrideScope then rosPkgs.overrideScope rosOverlay else rosPkgs
       ) rosPackages;
-    rosDistroOverlays = self: super: {
+    rosDistroOverlays = final: prev: {
       # Apply the overlay to multiple ROS distributions
-      rosPackages = applyDistroOverlay (import ./overlay.nix) super.rosPackages;
+      rosPackages = applyDistroOverlay (import ./overlay.nix) prev.rosPackages;
     };
 """
 ).strip()