darwin: add linux builder

flake.nix

@@ -5,7 +5,8 @@
     # Principle inputs
     nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
     flake-parts.url = "github:hercules-ci/flake-parts";
-    nix-darwin.url = "github:lnl7/nix-darwin";
+    # nix-darwin.url = "github:lnl7/nix-darwin";
+    nix-darwin.url = "github:LoganBarnett/nix-darwin/linux-builder-big-config"; # https://github.com/LnL7/nix-darwin/pull/878 (for 'systems')
     nix-darwin.inputs.nixpkgs.follows = "nixpkgs";
     home-manager.url = "github:nix-community/home-manager";
     home-manager.inputs.nixpkgs.follows = "nixpkgs";
@@ -113,6 +114,7 @@
             imports = [
               self.darwinModules.default # Defined in nix-darwin/default.nix
               ./systems/darwin.nix
+              ./systems/darwin/ci.nix
             ];
           };
           naivete = self.nixos-flake.lib.mkMacosSystem {

systems/darwin/ci.nix

@@ -0,0 +1,29 @@
+{ ... }:
+
+{
+  # TODO: GitHub Runners
+
+  # To build Linux derivations whilst on macOS.
+  # 
+  # NOTES:
+  # - To SSH, `sudo su -` and then `ssh -i /etc/nix/builder_ed25519  builder@linux-builder`.
+  #   Unfortunately, a simple `ssh linux-builder` will not work (Too many authentication failures).
+  # - To update virtualisation configuration, you have to disable, delete
+  #   /private/var/lib/darwin-builder/ and re-enable.
+  nix.linux-builder = {
+    enable = true;
+    systems = [
+      "x86_64-linux"
+      "aarch64-linux"
+    ];
+    config = { pkgs, lib, ... }: {
+      boot.binfmt.emulatedSystems = [ "x86_64-linux" ];
+      virtualisation = {
+        # Larger linux-builder cores, ram, and disk.
+        cores = 6;
+        memorySize = lib.mkForce (1024 * 16);
+        diskSize = lib.mkForce (1024 * 1); # In MB.
+      };
+    };
+  };
+}