internal scripts: use pinned, cached pkgs

Instead of setting up the script PATH via nix-shell, use
`nix shell` with inputs from the nix-bitcoin flake.
Advantages:
- Uses the nixpkgs version from the nix-bitcoin flake instead of
  `<nixpkgs>` from the user env (NIX_PATH), so the script runtime
  env is reproducible.
- The pkg derivations for the runtime env are cached, which greatly
  increases script startup speed.

This commit was generated by running the following script inside the
repo root dir:

def transform(path, src)
  if src =~ /#! *nix-shell +-i +bash +-p +(.*)/
    pkgs = $1
    if src =~ /^.*?(set -e.*?pipefail)\n/
      set_statement = $1
      src.sub!($&, '')
    end
    src.sub!(/\A.*?#! *nix-shell.*?\n/m, '')

    parents = ([ '..' ] * (path.split('/').count - 1)).join('/')

    [
      '#!/usr/bin/env bash',
      *set_statement,
      %(. "${BASH_SOURCE[0]%/*}/#{parents}/helper/run-in-nix-env" "#{pkgs}" "$@"),
      nil,
      src
    ].join("\n")
  end
end

Dir['**/*.sh'].each do |f|
  src = File.read(f)
  if new_src = transform(f, src)
    puts "Changed file #{f}"
    File.write(f, new_src)
  end
end

pkgs/python-packages/python-bitcointx/get-sha256.sh

@@ -1,6 +1,6 @@
-#! /usr/bin/env nix-shell
-#! nix-shell -i bash -p git gnupg
+#!/usr/bin/env bash
 set -euo pipefail
+. "${BASH_SOURCE[0]%/*}/../../../helper/run-in-nix-env" "git gnupg" "$@"
 
 TMPDIR=$(mktemp -d -p /tmp)
 trap 'rm -rf $TMPDIR' EXIT