The user's local node configuration directory usually contains a copy of
examples/shell.nix.
1. Move the shell implementation from shell.nix to nix-bitcoin/helper/makeShell.nix
Because the shell is no longer defined locally in the user's config
directory, we can now ship new shell features via nix-bitcoin updates.
2. Simplify examples/nix-bitcoin-release.nix
nix-bitcoin-release.nix, as generated via `fetch-release`, now
contains a simple fetchTarball statement which can be directly imported.
This allows us to get rid of the extra `nix-bitcoin-unpacked` derivation
which adds a dependency on the user's local nixpkgs.
To keep `fetch-release` as simple as possible for easy auditing, we just
fetch and verify a `nar-hash.txt` file that is now uploaded
via `push-release.sh`.
A migration guide for updating the user's local `shell.nix` is
automatically printed when the user starts a new shell after updating
nix-bitcoin.
This is achieved by throwing an error in `generate-secrets`, which is called
on shell startup.
This commit is required to deploy the new extensible `generate-secrets`
mechanism introduced in the next commit.
