nixos/plasma6: check `$XDG_CACHE_HOME` before `$HOME/.cache` for ksycoca cache wipe (#453299)

- `buildPythonPackage` and `buildPythonApplication` now default to `nix-update-script` as their default `updateScript`. This should improve automated updates, since nix-update is better maintained than the in-tree update script and has more robust fetcher support.

- `plasma6`: Fixed the `ksycoca` cache not being re-built when `$XDG_CACHE_HOME` is set to something that isn't `$HOME/.cache`.

## Nixpkgs Library {#sec-nixpkgs-release-25.11-lib}

<!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

  activationScript = ''

    # will be rebuilt automatically

    rm -fv "$HOME/.cache/ksycoca"*

    rm -fv "''${XDG_CACHE_HOME:-$HOME/.cache}/ksycoca"*

  '';

in

{