nixos/systemd-user: call systemd-tmpfiles during activation

Just like with system-wide tmpfiles, call `systemd-tmpfiles --create
--remove` for users during activation. This fixes an issue where new
entries in a user's tmpfiles are not reflected after activation, only at
boot when the user service systemd-tmpfiles-setup.service runs or only
after running systemd-tmpfiles manually.

nixos/modules/system/boot/systemd/user.nix

@@ -230,5 +230,9 @@ in {
           });
         })
         cfg.tmpfiles.users;
+
+    system.userActivationScripts.tmpfiles = ''
+      ${config.systemd.package}/bin/systemd-tmpfiles --user --create --remove
+    '';
   };
 }