ops/nixos: inject depot overlays into NIX_PATH as well

Opened by sterni at 2021-04-20T10·45+00

After [cl/3047](https://cl.tvl.fyi/c/depot/+/3047/), we make sure that pkgs for the configuration build comes from depot.third_party.nixpkgs with the overlays applied. However nix.nixPath only sets nixpkgs to pkgs.path which won't apply the overlays we have.

We'd need to set nixpkgs-overlays in NIX_PATH as well to correct this.