builders: run gc 3x per day

We can still adjust it if the disks fill up, but currently it is too frequent

services/baremetal-builder/default.nix

@@ -150,7 +150,7 @@ in
       script = "exec ${config.nix.package.out}/bin/nix-store --gc --store /mnt";
       serviceConfig.Type = "oneshot";
       serviceConfig.User = "builder";
-      startAt = "hourly";
+      startAt = "*-*-* 00/8:00:00";
     };
     systemd.timers.hydra-gc.timerConfig.Persistent = true;