Make nix-build args copy-pastable via `set -x`

Dependencies

• [2] SM5PVIIJ reproduce.tt: Use -I instead of $NIX_PATH
• [3] EE5GSP2L reproduce.tt: Wrap script to prevent problems piping into bash
• [4] 5Q4ZLL37 reproduce.tt: Add --help flag
• [5] HZWUT4YN Allow users to reproduce builds on their own systems
• [6] 3G7OYUSE reproduce.tt: Check for the presence of commands like nix-build
• [7] CRCZ62JS Fix passing arguments to the reproduction script
• [8] TL7KZWSU reproduce.tt: Use color
• [9] UROSQNYP reproduce.tt: Add --print-flags option

Change contents

• replacement in src/root/reproduce.tt at line 213
  [4.475]→[2.111:152](∅→∅),[2.152]→[4.3567:3568](∅→∅),[4.3567]→[4.3567:3568](∅→∅),[4.3568]→[4.441:487](∅→∅),[4.487]→[3.11:12](∅→∅)
  − echo "using these flags: ${args[@]}" >&2

  −

  − exec nix-build "${args[@]}" "${extraArgs[@]}"

  −
  [4.475]

  [3.12]
  + echo "using the following invocation:" >&2

  + set -x

  + nix-build "${args[@]}" "${extraArgs[@]}"