nix: flake: export under overlays.default

Keep the existing name to avoid breaking users, but per upstream, flakes
with a single overlay should export it under overlays.default rather
than under a top-level overlay attr.

flake.nix

@@ -5,8 +5,9 @@
   inputs.flake-utils.url = github:numtide/flake-utils;
 
   outputs = { self, nixpkgs, flake-utils }:
-    {
+    rec {
       overlay = final: prev: { inherit (self.packages.${final.system}) spacebar; };
+      overlays.default = overlay;
     }
     // flake-utils.lib.eachSystem [ "aarch64-darwin" "x86_64-darwin" ] (system:
       let pkgs = nixpkgs.legacyPackages.${system}; in