modules/builder: gc by percent used

modules/nixos/builder.nix

@@ -1,7 +1,21 @@
 {
-  imports = [ ../shared/builder.nix ];
+  config,
+  inputs,
+  pkgs,
+  ...
+}:
+{
+  nix.gc.automatic = false;
 
-  nix.gc.dates = "hourly";
+  systemd.services.free-space = {
+    serviceConfig.Type = "oneshot";
+    startAt = "hourly";
+    path = [
+      config.nix.package
+      pkgs.coreutils
+    ];
+    script = import "${inputs.self}/modules/shared/free-space.nix";
+  };
 
   # Bump the open files limit so that non-root users can run NixOS VM tests
   security.pam.loginLimits = [