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.

examples/shell.nix

@@ -1,53 +1,8 @@
 let
-  # This is either a path to a local nix-bitcoin source or an attribute set to
-  # be used as the fetchurl argument.
-  nix-bitcoin-release = import ./nix-bitcoin-release.nix;
-
-  nix-bitcoin-path =
-    if builtins.isAttrs nix-bitcoin-release then nix-bitcoin-unpacked
-    else nix-bitcoin-release;
-
-  nixpkgs-path = (import "${toString nix-bitcoin-path}/pkgs/nixpkgs-pinned.nix").nixpkgs;
-  pkgs = import nixpkgs-path {};
-  nix-bitcoin = pkgs.callPackage nix-bitcoin-path {};
-
-  nix-bitcoin-unpacked = (import <nixpkgs> {}).runCommand "nix-bitcoin-src" {} ''
-    mkdir $out; tar xf ${builtins.fetchurl nix-bitcoin-release} -C $out
-  '';
+  nix-bitcoin = toString (import ./nix-bitcoin-release.nix);
 in
-with pkgs;
-stdenv.mkDerivation rec {
-  name = "nix-bitcoin-environment";
-
-  path = lib.makeBinPath [ nix-bitcoin.extra-container ];
-
-  shellHook = ''
-    export NIX_PATH="nixpkgs=${nixpkgs-path}:nix-bitcoin=${toString nix-bitcoin-path}:."
-    export PATH="${path}''${PATH:+:}$PATH"
-
-    export NIX_BITCOIN_EXAMPLES_DIR="${toString ./.}"
-
-    fetch-release() {
-      ${toString nix-bitcoin-path}/helper/fetch-release
-    }
-
-    krops-deploy() {
-      # Ensure strict permissions on secrets/ directory before rsyncing it to
-      # the target machine
-      chmod 700 ${toString ./secrets}
-      $(nix-build --no-out-link ${toString ./krops/deploy.nix})
-    }
-
-    # Print logo if
-    # 1. stdout is a TTY, i.e. we're not piping the output
-    # 2. the shell is interactive
-    if [[ -t 1 && $- == *i* ]]; then
-      ${figlet}/bin/figlet "nix-bitcoin"
-    fi
-
-    (mkdir -p secrets; cd secrets; env -i ${nix-bitcoin.generate-secrets})
-
-    # Don't run this hook when another nix-shell is run inside this shell
-    unset shellHook
-  '';
-}
+  import "${nix-bitcoin}/helper/makeShell.nix" {
+    configDir = ./.;
+    # Set this to modify your shell
+    # extraShellInitCmds = (pkgs: ''<my bash code>'');
+  }