nixos/plotinus: fix evaluation

Apparently setting a variable via `environment.variables` when the same
is already present in `environment.sessionVariables` (that is merged
into the former option) creates a conflict.

For reference: this started with the change in #101274.

nixos/modules/programs/plotinus.nix

@@ -30,7 +30,7 @@ in
   ###### implementation
 
   config = mkIf cfg.enable {
-    environment.variables.XDG_DATA_DIRS = [ "${pkgs.plotinus}/share/gsettings-schemas/${pkgs.plotinus.name}" ];
+    environment.sessionVariables.XDG_DATA_DIRS = [ "${pkgs.plotinus}/share/gsettings-schemas/${pkgs.plotinus.name}" ];
     environment.variables.GTK3_MODULES = [ "${pkgs.plotinus}/lib/libplotinus.so" ];
   };
 }