Tweak bat options

Dependencies

• [2] DRXLMZ7L Replace pygmentize with bat
• [3] FDN24OK3 Remove workarounds for open nixpkgs PRs
• [4] 4A2DSMIM Clean up nix-container-demo-helper.sh
• [5] W2G433QU Refactor libdemo and flakeref for nix run commands
• [6] DBSDTDTD Keep nixpkgs source(s) as gcroots

Change contents

• replacement in bash/nix-container-demo-helper.sh at line 12
  [3.726]→[2.172:260](∅→∅)
  − COLORTERM="" TERM="" nix run "${NIXPKGS_FLAKEREF}#bat" -- --pager none "$@"
  [3.726]

  [3.1244]
  + nix run "${NIXPKGS_FLAKEREF}#bat" -- --pager none --decorations always --color=always "$@"
• replacement in bash/nix-container-demo-helper.sh at line 17
  [3.1331]→[3.1331:1352](∅→∅)
  − cat "$@"
  [3.1331]

  [3.1352]
  + nix run "${NIXPKGS_FLAKEREF}#bat" -- --pager none --decorations always "$@"