Are some of your systems a little bit behind? Did you run into this recently?

```
$ nix-env -iA nixpkgs.nix
replacing old 'nix-2.1pre6148_a4aac7f'
installing 'nix-2.1.1'
building '/nix/store/7ln58dlcma4biz36k86jfn7ilajrv9a2-user-environment.drv'...
error: unsupported builtin function 'buildenv'
```

I think it stands to reason that the commit that introduced Nix 2.1 should be relying on Nix 2.1 constructs, so I'm pretty confident this should be a good interim step:

```
$ nix-env -f '<nixpkgs>' -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/347638ccfee74c06cae53983ea741d39d1efc032.tar.gz -iA nix
```