[demo@nixos:~]$ git clone --single-branch --branch ungoogled https://github.com/squalus/nixpkgs.git
Cloning into 'nixpkgs'...
remote: Enumerating objects: 16, done.
remote: Counting objects: 100% (16/16), done.
remote: Compressing objects: 100% (15/15), done.
remote: Total 1637069 (delta 1), reused 10 (delta 1), pack-reused 1637053
Receiving objects: 100% (1637069/1637069), 932.87 MiB | 6.57 MiB/s, done.
Resolving deltas: 100% (1116580/1116580), done.
Updating files: 100% (20643/20643), done.

[demo@nixos:~]$ export NIXPKGS=~/nixpkgs

[demo@nixos:~]$ ls ~/nixpkgs/pkgs/applications/networking/browsers/chromium-ungoogled
browser.nix  common.nix  default.nix  patches  plugins.nix  ungoogled.nix  ungoogled-src.nix  update.nix  update.sh  upstream-info.nix

[demo@nixos:~]$ nix-env -f $NIXPKGS -i chromium-ungoogled
error: selector 'chromium-ungoogled' matches no derivations

[demo@nixos:~]$ cat /etc/issue

<<< Welcome to NixOS 20.03pre209250.7184df6beb8 (\m) - \l >>>