nix: don't use deprecated pkgs.system (#9458)

'system' has been renamed to/replaced by 'stdenv.hostPlatform.system'

NixOS/nixpkgs#456527

Author: mitchellh

flake.nix

@@ -107,10 +107,10 @@
       overlays = {
         default = self.overlays.releasefast;
         releasefast = final: prev: {
-          ghostty = self.packages.${prev.system}.ghostty-releasefast;
+          ghostty = self.packages.${prev.stdenv.hostPlatform.system}.ghostty-releasefast;
         };
         debug = final: prev: {
-          ghostty = self.packages.${prev.system}.ghostty-debug;
+          ghostty = self.packages.${prev.stdenv.hostPlatform.system}.ghostty-debug;
         };
       };
       create-vm = import ./nix/vm/create.nix;