nixos/homepage-dashboard: fix stale cache issue

When the cache is stale, homepage-dashboard serves bad data (UI lacks
styling etc.). This issue happens at least on homepage-dashboard version
upgrades.

Fixes #346016.

(cherry picked from commit 58f12ee)

nixos/modules/services/misc/homepage-dashboard.nix

@@ -254,6 +254,14 @@ in
           ExecStart = lib.getExe cfg.package;
           Restart = "on-failure";
         };
+
+        preStart = ''
+          # Related:
+          # * https://github.com/NixOS/nixpkgs/issues/346016 ("homepage-dashboard: cache dir is not cleared upon version upgrade")
+          # * https://github.com/gethomepage/homepage/discussions/4560 ("homepage NixOS package does not clear cache on upgrade leaving broken state")
+          # * https://github.com/vercel/next.js/discussions/58864 ("Feature Request: Allow configuration of cache dir")
+          rm -rf "$NIXPKGS_HOMEPAGE_CACHE_DIR"/*
+        '';
       };
 
       networking.firewall = lib.mkIf cfg.openFirewall {