nixos/tailscale: add package as an option
nixos/tailscale: add package as an option

This simplifies testing changes to the tailscale service on a local
machine. You can use this as such:

```nix
let
  tailscale_patched = magic {};
in {
  services.tailscale = {
    enable = true;
    package = tailscale_patched;
  };
};
```

Signed-off-by: Christine Dodrill <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@christine.website>

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