Use nix-systems

Not hugely important because it only affects our packages, not overlay
or home-manager module, but might as well.

flake.lock

@@ -71,7 +71,8 @@
       "inputs": {
         "doomemacs": "doomemacs",
         "emacs-overlay": "emacs-overlay",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "systems": "systems_2"
       }
     },
     "systems": {
@@ -88,6 +89,21 @@
         "repo": "default",
         "type": "github"
       }
+    },
+    "systems_2": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
     }
   },
   "root": "root",

flake.nix

@@ -15,6 +15,7 @@
 {
   inputs = {
     nixpkgs.url = "nixpkgs";
+    systems.url = "github:nix-systems/default";
     doomemacs = {
       url = "github:doomemacs/doomemacs";
       flake = false;
@@ -29,9 +30,11 @@
     };
   };
 
-  outputs = { doomemacs, nixpkgs, emacs-overlay, ... }: let
-    systems = [ "x86_64-linux" ];
-    perSystemPackages = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
+  outputs = { systems, doomemacs, nixpkgs, emacs-overlay, ... }: let
+    perSystemPackages = let
+      eachSystem = nixpkgs.lib.genAttrs (import systems);
+    in
+      f: eachSystem (system: f nixpkgs.legacyPackages.${system});
     # Hack to avoid pkgs.extend having to instantiate an additional nixpkgs.
     #
     # We need emacsPackagesFor from the overlay, but neither the overlay itself