improve examples/shell.nix

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.

flake.nix

@@ -62,7 +62,7 @@
         nbPkgs = self.mkNbPkgs { inherit system pkgs; };
 
         packages = flake-utils.lib.flattenTree (removeAttrs nbPkgs [
-          "pinned" "modulesPkgs" "nixops19_09" "krops"
+          "pinned" "modulesPkgs" "nixops19_09" "krops" "generate-secrets"
         ]) // {
           runVM = mkVMScript packages.vm;