nixos/release*.nix: Clean nixpkgs sources by default

Currently, when building NixOS from a git clone, Nix has to copy
the entire repo at >1GB into the store by default. That is not
necessary and causes a dumping large path message.
If you need the old behaviour for some reason, you will have to
specify it by passing the path to your repo explicitly as the
nixpkgs argument like this:

  --arg nixpkgs '{outPath = ./.; revCount = 56789; shortRev = "gfedcba"; }'