Add nix-bitcoin

nix-bitcoin is a collection of Nix packages and NixOS modules for easily installing full-featured Bitcoin nodes with an emphasis on security.

Now that Sovereign Stack is based on Incus, we can spawn NixOS VMs as the base OS. The next logical addition to Sovereign Stack, then, is nix-bitcoin, to serve as the most secure option for running a Bitcoin full node in Sovereign Stack. This would be in addition to the docker engine-based btcpayserver (currently most secure option) and lnplay (for use in lab environments).

There may be another opportunity here. Maybe NixOS is installed on the bare-metal management machine along with the latest version of incus.