does anyone know how to enable microcode in nixOS? it's not included in the generated boot entry
```
title NixOS
version Generation 55 NixOS 19.09pre184290.460136f8a3a, Linux Kernel 5.1.15, Built on 2019-07-02
linux /efi/nixos/a9sdxnk7wm21xxsj29h7blvizx4b0lwx-linux-5.1.15-bzImage.efi
initrd /efi/nixos/rvwxaj0n60mjdlmkpgscai7jvh8f18am-initrd-linux-5.1.15-initrd.efi
options systemConfig=/nix/store/qjqc384w2nqxjchrqh4gv2hmdaz7n17r-nixos-system-nixos-19.09pre184290.460136f8a3a init=/nix/store/qjqc384w2nqxjchrqh4gv2hmdaz7n17r-nixos-system-nixos-19.09pre184290.460136f8a3a/init loglevel=4
machine-id d72ada09304e4a8fa09d4daa6aaae3eb
```