examples/shell.nix: minor improvements

- Use idiomatic var name `pkgs` for the imported nixpkgs.
- Don't add `figlet` to PATH because it's only used internally.
- Only print figlet in interactive shells to avoid interfering with
  stdout when running `nix-shell --run <cmd>`.
- Define `fetch-release` as a function to enable running it via
  `nix-shell --run fetch-release`

examples/shell.nix

@@ -8,19 +8,18 @@ let
     else nix-bitcoin-release;
 
   nixpkgs-path = (import "${toString nix-bitcoin-path}/pkgs/nixpkgs-pinned.nix").nixpkgs;
-  nixpkgs = import nixpkgs-path {};
-  nix-bitcoin = nixpkgs.callPackage nix-bitcoin-path {};
+  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
   '';
 in
-with nixpkgs;
-
+with pkgs;
 stdenv.mkDerivation rec {
   name = "nix-bitcoin-environment";
 
-  path = lib.makeBinPath [ nix-bitcoin.extra-container figlet ];
+  path = lib.makeBinPath [ nix-bitcoin.extra-container ];
 
   shellHook = ''
     export NIX_PATH="nixpkgs=${nixpkgs-path}:nix-bitcoin=${toString nix-bitcoin-path}:."
@@ -28,7 +27,9 @@ stdenv.mkDerivation rec {
 
     export NIX_BITCOIN_EXAMPLES_DIR="${toString ./.}"
 
-    alias fetch-release="${toString nix-bitcoin-path}/helper/fetch-release"
+    fetch-release() {
+      ${toString nix-bitcoin-path}/helper/fetch-release
+    }
 
     krops-deploy() {
       # Ensure strict permissions on secrets/ directory before rsyncing it to
@@ -37,7 +38,13 @@ stdenv.mkDerivation rec {
       $(nix-build --no-out-link ${toString ./krops/deploy.nix})
     }
 
-    figlet "nix-bitcoin"
+    # 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