nixos/image/repart-verity-store: mount nix-store in the module

You always need this for the image to be bootable so lets
do it in the image and not in the test

nixos/modules/image/repart-verity-store.nix

@@ -89,6 +89,12 @@ in
       };
     };
 
+    fileSystems."/nix/store" = lib.mkDefault {
+      device = "/usr/nix/store";
+      fsType = "none";
+      options = [ "bind" ];
+    };
+
     image.repart.partitions = {
       # dm-verity hash partition
       ${cfg.partitionIds.store-verity}.repartConfig = {

nixos/tests/appliance-repart-image-verity-store.nix

@@ -18,18 +18,12 @@
     {
       imports = [ ../modules/image/repart.nix ];
 
-      virtualisation.fileSystems = lib.mkVMOverride {
+      virtualisation.fileSystems = lib.mkVMOverride { };
+      fileSystems = {
         "/" = {
           fsType = "tmpfs";
           options = [ "mode=0755" ];
         };
-
-        # bind-mount the store
-        "/nix/store" = {
-          device = "/usr/nix/store";
-          fsType = "none";
-          options = [ "bind" ];
-        };
       };
 
       image.repart = {