nixos/plasma6: check $XDG_CACHE_HOME before $HOME/.cache for ksycoca cache wipe

The previous implementation breaks if `$XDG_CACHE_HOME` is set to
anything other than what is expected, since KDE will now write the cache
there instead. Some users set `$XDG_CACHE_HOME` to something like
`$HOME/.local/cache` to de-clutter `$HOME`. In such case, the cache won't
be found and removed, which will cause the KDE runner to not be able to
launch anything. It will now work regardless.

doc/release-notes/rl-2511.section.md

@@ -298,6 +298,8 @@
 
 - `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. -->

nixos/modules/services/desktop-managers/plasma6.nix

@@ -20,7 +20,7 @@ let
 
   activationScript = ''
     # will be rebuilt automatically
-    rm -fv "$HOME/.cache/ksycoca"*
+    rm -fv "''${XDG_CACHE_HOME:-$HOME/.cache}/ksycoca"*
   '';
 in
 {