NixOS seed node setup

Deploy

Update the SSH key in modules/users.nix ssh_key to one of your own.
Stage or commit the modification using git to include the modification in the build:
```
git add modules/users.nix
git commit -m "update ssh key" # (optional)
```

$ nix-shell -p nixos-anywhere
[nix-shell:~]$ nixos-anywhere --flake .#ax52 root@<ip_address>

$ nix-shell -p nixos-rebuild
[nix-shell:~]$ nixos-rebuild switch --flake .#ax52 --target-host root@<ip_address>

The bitcoind service will be running on mainnet.