feat(nixbuild): centralize distributed build config

- Moved shared distributed build settings to `nixbuild.nix` for reuse.
- Updated `m4` and `rialo` systems to import the centralized configuration.
- Simplifies maintenance and ensures consistency across systems.

systems/aarch64-darwin/m4/default.nix

@@ -4,45 +4,10 @@ with lib.metacfg;
 {
   system.defaults.dock.autohide = false;
 
-  nix.distributedBuilds = true;
-
-  nix.buildMachines = [
-    {
-      hostName = "m1.local";
-      system = "aarch64-darwin";
-      maxJobs = 4;
-      speedFactor = 2;
-      sshUser = "harald";
-      sshKey = "/etc/ssh/nix-builder-key";
-    }
-    {
-      hostName = "rialo.local";
-      system = "aarch64-darwin";
-      maxJobs = 4;
-      speedFactor = 2;
-      sshUser = "harald";
-      sshKey = "/etc/ssh/nix-builder-key";
-    }
-    {
-      hostName = "sgx.local";
-      system = "x86_64-linux";
-      maxJobs = 4;
-      speedFactor = 1;
-      sshUser = "harald";
-      sshKey = "/etc/ssh/nix-builder-key";
-    }
+  imports = [
+    ./../../nixbuild.nix
   ];
 
-  nix.settings = {
-    substituters = [
-      "https://cache.nixos.org"
-      "ssh-ng://harald@m4.local?ssh-key=/etc/ssh/nix-builder-key"
-      "ssh-ng://harald@rialo.local?ssh-key=/etc/ssh/nix-builder-key"
-    ];
-
-    builders-use-substitutes = true;
-  };
-
   homebrew = {
     enable = true;
     brews = [