Recommended deployment method: Use Flakes, improve performance

[erikarvstedt]

Use Flakes for improved extensibility and pure builds.
Switch to a faster deployment scheme.

This requires Nix PR Source tree abstraction to be merged:
nix-bitcoin config flakes potentially include plaintext secrets files. PR Source tree abstraction allows evaluating the flake without copying its source (including secrets) to the store.

Note:
nix-bitcoin configs can already use flakes if they encrypt secrets or store them outside of the repo or if they generate secrets on the target node (nix-bitcoin.generateSecrets = true).

[prusnak]

This requires Nix PR Source tree abstraction to be merged:

Here is the mentioned Nix PR: NixOS/nix#6530