Common/nix: add remote builder

roles/common/programs/nix.nix

@@ -1,9 +1,38 @@
-{systemPkgs, ...}: {
+{
+  systemPkgs,
+  config,
+  flakeSelf,
+  ...
+}: {
+  age.secrets = {
+    remoteBuilderKey.file = "${flakeSelf.inputs.secrets}/WinMax2/nixrbld-private-key.age";
+  };
+
+  programs.ssh = {
+    knownHosts.winmax2.publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIPE+ksvEq/I2LMLOztVXpLE9yuI6EkRh4EtXdlYkhl6C WinMax2 host key";
+    extraConfig = ''
+      Host nixrbld
+        HostName winmax2
+        IdentitiesOnly yes
+        IdentityFile ${config.age.secrets.remoteBuilderKey.path}
+        User nixrbld
+    '';
+  };
+
   nix = {
     settings = {
       auto-optimise-store = true;
       experimental-features = "nix-command flakes";
     };
+    distributedBuilds = true;
+    buildMachines = [
+      {
+        hostName = "nixrbld";
+        system = "x86_64-linux";
+        protocol = "ssh-ng";
+        maxJobs = 4;
+      }
+    ];
     optimise = {
       automatic = true;
       dates = ["weekly"];