examples/shell.nix: don't run shellHook on subsequent nix-shells

This avoids an extra delay and the unexpected creation of secrets when
run in another dir.

Needed for the 'fetch-release' script introduced in a later commit.

examples/shell.nix

@@ -32,5 +32,8 @@ stdenv.mkDerivation rec {
 
     figlet "nix-bitcoin"
     (mkdir -p secrets; cd secrets; ${nix-bitcoin.generate-secrets})
+
+    # Don't run this hook when another nix-shell is run inside this shell
+    unset shellHook
   '';
 }