Preventing Nix from upgrading git to git-full

I'm using Nix on Ubuntu to have more modern versions of some packages, one of them being git. When I'm run nix-env -u, this is what happens: % nix-env -u --dry-run '*' (dry run; not doing anything) upgrading git-1.8.5.4' to git-1.9.0-full' This is not what I want - I prefer the plain version git-1.9.0 over the -full version. I know I can manually upgrade to the plain version with nix-env -i git-1.9.0, but is there a way to make nix-env -u to do what I want?