cloned tailscaled.state files lead to network flakiness

The easiest way to add a new Raspberry Pi Zero to your house is to clone the SD card of an existing Pi. For most purposes this works well.

Unfortunately, if the Pi is already running tailscale, that means the new device gets a copy of `/var/lib/tailscale/tailscaled.state`. It uses the same private key as the old device to ask for the same IP address, and both devices start fighting over it. This has several negative consequences:

- the Pi Zero uses 20-40% of its CPU constantly sending netmaps and reconnecting to DERP
- machines can reach each other sometimes, but not always
- otherwise well-behaved machines on the networks receive constant netmap updates

The experience is worse than if the control server expired the key and took both devices off the network.