modules: move builder into common

modules/nixos/common/builder.nix

@@ -0,0 +1,32 @@
+{
+  config,
+  inputs,
+  lib,
+  pkgs,
+  ...
+}:
+{
+  config = lib.mkIf (lib.hasPrefix "build" config.networking.hostName) {
+    nix.gc.automatic = false;
+
+    systemd.services.free-space = {
+      serviceConfig.Type = "oneshot";
+      startAt = "hourly";
+      path = [
+        config.nix.package
+        pkgs.coreutils
+      ];
+      script = builtins.readFile "${inputs.self}/modules/shared/free-space.bash";
+    };
+
+    # Bump the open files limit so that non-root users can run NixOS VM tests
+    security.pam.loginLimits = [
+      {
+        domain = "*";
+        item = "nofile";
+        type = "-";
+        value = "20480";
+      }
+    ];
+  };
+}