Keep nixpkgs source(s) as gcroots

Dependencies

• [2] 4A2DSMIM Clean up nix-container-demo-helper.sh
• [*] M5U5PAX7 Add wrapper script, improve libdemo
• [*] JGZ7GBCL Add linting
• [*] HCY72TQR Improve linting
• [*] W2G433QU Refactor libdemo and flakeref for nix run commands

Change contents

• edit in demo-full.sh at line 17
  [5.222]

  [6.376]
  + __nix_make_gcroot "$(nix flake metadata "${NIXPKGS_FLAKEREF}" | grep 'Path' | awk '{ print $2; }')"
• edit in bash/nix-container-demo-helper.sh at line 15
  [2.266]

  [7.1244]
  + __nix_make_gcroot "$(nix flake metadata "github:sfrijters/nixpkgs/pygments-mainprogram" | grep 'Path' | awk '{ print $2; }')"
• edit in bash/nix-container-demo-helper.sh at line 29
  [2.451]

  [7.1828]
  + __nix_make_gcroot "$(nix flake metadata "github:sfrijters/nixpkgs/tiv-add-wrapper" | grep 'Path' | awk '{ print $2; }')"