Turn automatic nix garbage collection off in microvm

modules/c3d2.nix

@@ -397,7 +397,7 @@ in
           );
         };
         gc = {
-          automatic = true;
+          automatic = lib.mkDefault true;
           dates = "06:00";
           options = "--delete-older-than 21d";
           randomizedDelaySec = "6h";

modules/microvm.nix

@@ -167,5 +167,8 @@ in
 
     # autoupdates do not make sense inside MicroVMs with read-only /nix/store
     c3d2.autoUpdate = false;
+
+    # nix store is mounted read only
+    nix.gc.automatic = false;
   };
 }