Add xdg settings, cache, cleanup script

Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>

hosts/eirene/format.nix

@@ -58,6 +58,10 @@
                     mountpoint = "/nix";
                     mountOptions = ["subvol=nix" "compress=zstd" "noatime"];
                   };
+                  "/cache" = {
+                    mountpoint = "/cache";
+                    mountOptions = ["subvol=cache" "compress=zstd" "noatime"];
+                  };
                 };
               };
             };