Nix flake: Use eachDefaultSystem from flake-utils.

We were doing the same thing and it exists, so why not use it?

Change-Id: I401e15c3a5d1a5ae0fee3b99d3e186547f1de2f6
diff --git a/flake.nix b/flake.nix
index 1979081..973036e 100644
--- a/flake.nix
+++ b/flake.nix
@@ -12,7 +12,7 @@
   };
 
   outputs = inputs@{ self, nixpkgs, flake-utils, ... }:
-    flake-utils.lib.eachSystem [ "x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin" ] (system:
+    flake-utils.lib.eachDefaultSystem (system:
       let
         overlays = [ ];