Fix unbound variable error

Dependencies

• [2] U6EHWR7Y Tweak --gcroots
• [3] HCY72TQR Improve linting
• [4] WNFTZY3M Optionally prevent relevant nix store paths from being GC'ed
• [*] M5U5PAX7 Add wrapper script, improve libdemo

Change contents

• replacement in demo-full.sh at line 52
  [2.345]→[2.345:407](∅→∅)
  − echo "Unknown or missing --gcroots option '$2'"
  [2.345]

  [2.407]
  + echo "Unknown or missing --gcroots option '${2:-}'"