feat(aya): qemu-aarch64 builder (disabled)

`binfmt_misc` does not currently work in containers...

modules/extern/nixos/nix.nix

@@ -10,6 +10,7 @@
   inherit (gensokyo-zone.lib) unmerged mkAlmostOptionDefault;
   cfg = config.gensokyo-zone.nix;
   nixModule = {
+    lib,
     gensokyo-zone,
     nixosConfig,
     nixosOptions,
@@ -17,6 +18,7 @@
     ...
   }: let
     inherit (gensokyo-zone.lib) unmerged domain;
+    inherit (lib.modules) mkOptionDefault;
     inherit (nixosConfig.gensokyo-zone) access;
   in {
     options = with lib.types; {
@@ -31,6 +33,10 @@
       };
       builder = {
         enable = mkEnableOption "aya nixbld remote builder";
+        cross = {
+          aarch64 = mkEnableOption "qemu-aarch64";
+          armv7l = mkEnableOption "qemu arm";
+        };
         domain = mkOption {
           type = str;
           default = "nixbld.${domain}";
@@ -98,6 +104,10 @@
         })
       ];
       builder = {
+        systems = mkMerge [
+          (mkIf config.builder.cross.aarch64 (mkOptionDefault [ "aarch64-linux" ]))
+          (mkIf config.builder.cross.armv7l (mkOptionDefault [ "armv7l-linux" ]))
+        ];
         domain = mkMerge [
           (mkIf access.tail.enabled (mkAlmostOptionDefault "nixbld.tail.${domain}"))
           (mkIf access.local.enable (mkDefault "nixbld.local.${domain}"))

nixos/cross/aarch64.nix

@@ -0,0 +1,30 @@
+{ pkgs, config, lib, ... }: let
+  inherit (lib.options) mkEnableOption mkPackageOption;
+  inherit (lib.modules) mkIf;
+  cfg = config.boot.binfmt.cross.aarch64;
+in {
+  options = {
+    boot.binfmt.cross.aarch64 = {
+      enable = mkEnableOption "qemu-aarch64" // {
+        default = true;
+      };
+      package = mkPackageOption pkgs "qemu" { };
+      armv7l = mkEnableOption "arm.cachix.org";
+    };
+  };
+
+  config = {
+    boot.binfmt = {
+      emulatedSystems = mkIf cfg.enable [ "aarch64-linux" ];
+      registrations.aarch64-linux = mkIf cfg.enable {
+        interpreter = "${cfg.package}/bin/qemu-aarch64";
+        wrapInterpreterInShell = false;
+      };
+    };
+
+    nix.settings = mkIf cfg.armv7l {
+      substituters = [ "https://arm.cachix.org/" ];
+      trusted-public-keys = [ "arm.cachix.org-1:5BZ2kjoL1q6nWhlnrbAl+G7ThY7+HaBRD9PZzqZkbnM=" ];
+    };
+  };
+}

systems/aya/nixos.nix

@@ -6,6 +6,7 @@
     nixos.base
     nixos.reisen-ct
     nixos.nixbld
+    #nixos.cross.aarch64
     nixos.tailscale
     nixos.github-runner.zone
     nixos.minecraft.bedrock