hardware.nix (snippet):
...
  hardware.deviceTree = {
    overlays = [
      ./dtb-overlays/overclock.dtbo
    ];
  };
...


File in question:

➜ ls -al dtb-overlays
total 12
drwxr-xr-x 2 root root 4096 Jul 31 21:47 ./
drwxr-xr-x 5 root root 4096 Jul 31 22:15 ../
-rw-r--r-- 1 root root  425 Jul 31 21:47 overclock.dtbo


Error Message:

sudo nixos-rebuild build --show-trace
building Nix...
building the system configuration...
these derivations will be built:
  /nix/store/s87y9z1vik8hphaj9zmph97rar6fzmby-device-tree-overlays.drv
  /nix/store/sl81f30jczfknm36ffl0m2lrlbzn4y8v-nixos-system-serenity-20.09pre236091.28fce082c8c.drv
building '/nix/store/s87y9z1vik8hphaj9zmph97rar6fzmby-device-tree-overlays.drv'...
Couldn't open blob from '/etc/nixos/dtb-overlays/overclock.dtbo': No such file or directory

Failed to read '/etc/nixos/dtb-overlays/overclock.dtbo'
builder for '/nix/store/s87y9z1vik8hphaj9zmph97rar6fzmby-device-tree-overlays.drv' failed with exit code 1
cannot build derivation '/nix/store/sl81f30jczfknm36ffl0m2lrlbzn4y8v-nixos-system-serenity-20.09pre236091.28fce082c8c.drv': 1 dependencies couldn't be built
error: build of '/nix/store/sl81f30jczfknm36ffl0m2lrlbzn4y8v-nixos-system-serenity-20.09pre236091.28fce082c8c.drv' failed