The word-picking one-liner used `eval echo \${...}`, but in a Nix
indented string a backslash does not escape `${` -- it starts
interpolation, so evaluation failed with "syntax error, unexpected
invalid token". Replace the eval/positional-parameter trick with a
plain `shuf` pipeline that contains no `${` at all.
Found by nixos-rebuild boot on staging-dev. The shell logic itself was
tested; the file had not been parse-checked by Nix. Re-scanned the
whole module: every remaining `${` is an intended interpolation and
every `''` is a string delimiter.