added cache to llama swap

modules/llama-swap.nix

@@ -9,6 +9,7 @@
 
   services.llama-swap = {
     enable = true;
+    package = pkgs.unstable.llama-swap;
     port = 8085;
     openFirewall = true;
 
@@ -71,4 +72,18 @@
 
       };
   };
+
+  # llama.cpp tries to create its cache under $HOME/.cache; when launched as a
+  # system service HOME may default to "/" ("//.cache/..."), which is often
+  # read-only. Give it a writable cache/state location.
+  systemd.services.llama-swap.serviceConfig = {
+    StateDirectory = "llama-swap";
+    CacheDirectory = "llama-swap";
+    Environment = [
+      "HOME=/var/lib/llama-swap"
+      "XDG_CACHE_HOME=/var/cache/llama-swap"
+      "MESA_SHADER_CACHE_DIR=/var/cache/llama-swap/mesa"
+      "MESA_SHADER_CACHE_MAX_SIZE=1G"
+    ];
+  };
 }