nixos/frigate: don't clear model cache

Upstream stores the model cache in the config directory, which is
unnecessarily messy. The cache directory is still the correct place for
these, since they can be pruned and redownloaded, we just don't want it
to happen on every restart.

Fixes: #427714
(cherry picked from commit cb4fd4e3ca)

nixos/modules/services/video/frigate.nix

@@ -632,7 +632,8 @@ in
       serviceConfig = {
         ExecStartPre = [
           (pkgs.writeShellScript "frigate-clear-cache" ''
-            rm --recursive --force /var/cache/frigate/*
+            shopt -s extglob
+            rm --recursive --force /var/cache/frigate/!(model_cache)
           '')
           (pkgs.writeShellScript "frigate-create-writable-config" ''
             cp --no-preserve=mode "${format.generate "frigate.yml" filteredConfig}" /run/frigate/frigate.yml