Seems to have been changed in https://github.com/NixOS/nixpkgs/pull/259891
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6I2HQYU2ZU5STWSYDZV3PLK6J3J4DWKXIJEZ3CCOVPXUJRR627BAC
services.xserver = {
services.xserver.xkb = {
xkbVariant = "";
variant = "";