flake: support aarch64-linux

...I don't know how to remove the mention of x86_64-linux for colmena,
or if it actually matters, so I'm just leaving that there for now.

flake.nix

@@ -25,7 +25,13 @@
 
   outputs = { self, nixpkgs, terranix, colmena, ... } @ inputs:
   let
-    system = "x86_64-linux";
+    supportedSystems = [ "x86_64-linux" "aarch64-linux" ];
+    forEachSystem = f: builtins.listToAttrs (map (system: {
+      name = system;
+      value = f system;
+    }) supportedSystems);
+    systemBits = forEachSystem (system: rec {
+      inherit system;
       pkgs = import nixpkgs {
         localSystem = system;
         overlays = [
@@ -34,7 +40,6 @@
           inputs.nix-gerrit.overlays.default
         ];
       };
-    lib = pkgs.lib;
       terraform = pkgs.opentofu;
       terraformCfg = terranix.lib.terranixConfiguration {
         inherit system;
@@ -46,9 +51,12 @@
           }
         ];
       };
+    });
+    forEachSystem' = f: forEachSystem (system: (f systemBits.${system}));
+    inherit (nixpkgs) lib;
   in
   {
-    apps.${system} = {
+    apps = forEachSystem' ({ system, pkgs, terraformCfg, terraform, ... }: {
       tf = {
         type = "app";
         program = toString (pkgs.writers.writeBash "tf" ''
@@ -59,9 +67,10 @@
       };
 
       default = self.apps.${system}.tf;
-    };
+    });
 
-    devShells.${system}.default = pkgs.mkShell {
+    devShells = forEachSystem' ({ system, pkgs, ... }: {
+      default = pkgs.mkShell {
         packages = [
           inputs.agenix.packages.${system}.agenix
 
@@ -70,6 +79,7 @@
           (pkgs.callPackage ./lib/colmena-wrapper.nix { })
         ];
       };
+    });
 
     nixosConfigurations = (colmena.lib.makeHive self.outputs.colmena).nodes;
 
@@ -91,14 +101,7 @@
 
       builders = lib.listToAttrs (lib.genList makeBuilder 12);
     in {
-      meta.nixpkgs = import nixpkgs {
-        localSystem = system;
-        overlays = [
-          inputs.hydra.overlays.default
-          inputs.lix.overlays.default
-          inputs.nix-gerrit.overlays.default
-        ];
-      };
+      meta.nixpkgs = systemBits.x86_64-linux.pkgs;
       meta.specialArgs.inputs = inputs;
 
       bagel-box.imports = commonModules ++ [ ./hosts/bagel-box ];