add auto gc

closed #7

modules/crans/default.nix

@@ -17,6 +17,7 @@ in
     ./nullmailer.nix
     ./packages.nix
     ./ssh.nix
+    ./store.nix
     ./users.nix
     ./virtualisation.nix
   ];

modules/crans/store.nix

@@ -0,0 +1,10 @@
+{...}:
+{
+  nix.gc = {
+    automatic = true;
+    dates = "04:15";
+    # avoid to run nix gc on all machine at the same time
+    randomizedDelaySec = "1h";
+    options = "--delete-older-than 14d";
+  };
+}