Use emacs-overlay to install Emacs

- we use the bleeding edge emacs master branch so beware when updating

- revision fb1cdbb0a12d7f0e0e50022c405aca7c856dd233 of emacs-overlay
  seems to work quite well.

flake.nix

@@ -13,6 +13,9 @@
       inputs.nixpkgs-22_11.follows = "nixpkgs";
     };
 
+    emacs.url = "github:nix-community/emacs-overlay";
+    emacs.inputs.nixpkgs.follows = "nixpkgs";
+
     # age for nix to store encrypted passwords conveniently
     agenix = {
       url = "github:ryantm/agenix";
@@ -24,7 +27,12 @@
     nixosConfigurations."thrall" = nixpkgs.lib.nixosSystem {
       system = "x86_64-linux";
       modules = [
-        ({ nixpkgs = { config.allowUnfree = true; }; })
+        ({
+          nixpkgs = {
+            config.allowUnfree = true;
+            overlays = with inputs; [ emacs.overlay ];
+          };
+        })
         snm.nixosModule
         ./modules/security.nix
         ./hosts/thrall