ci: Format `nixfmt` command prominently (#373939)

          if (( "${#unformattedFiles[@]}" > 0 )); then

            echo "Some new/changed Nix files are not properly formatted"

            echo "Please format them using the Nixpkgs-specific \`nixfmt\` by going to the Nixpkgs root directory, running \`nix-shell\`, then:"

            echo

            echo "nixfmt ${unformattedFiles[*]@Q}"

            echo "Make sure your branch is up to date with master, rebase if not."

            echo

            echo "Make sure your branch is up to date with master; rebase if not."

            echo "If you're having trouble, please ping @NixOS/nix-formatting"

            exit 1

          fi