https://github.com/actions-runner-controller/actions-runner-... works quite well with its autoscaling. You could create a zonal GKE cluster, which is in free tier, and create a small spot vm node pool with ARM nodes. It wouldn't be entirely free but it would cost quite low amount of money.

You realistically want something with ARMv8.2 or better, these are relatively easy to acquire with the current supply chains, they're beefy, can be equipped with fast storage, and they are both small units you can put on your desk. Note that the Xavier will require you to fiddle with the usual Nvidia bullshit through their SDK but it's otherwise a standard Ubuntu machine. It should be possible to get another distro on there too. The M1 will almost certainly perform better overall watt-for-watt though.