nixos/terminfo: Improve snippet generating the “all terminfo” list

- Avoid false-positives on package sets that contain a `terminfo` derivation,
like `haskellPackages` and `sbclPackages`.
- Directly provide a list of names that can be used to update the NixOS module,
rather than a list of derivations which is hard to read in the REPL.

nixos/modules/config/terminfo.nix

···
 
   config = {
 
-    # can be generated with: filter (drv: (builtins.tryEval (drv ? terminfo)).value) (attrValues pkgs)
+    # can be generated with:
+    # attrNames (filterAttrs
+    #  (_: drv: (builtins.tryEval (isDerivation drv && drv ? terminfo)).value)
+    #  pkgs)
     environment.systemPackages = mkIf config.environment.enableAllTerminfo (map (x: x.terminfo) (with pkgs; [
       alacritty
       foot