Previously I got the following error message:
```
error: opening file '/home/ma27/Projects/nixpkgs/nixos/modules/installer/default.nix': No such file or directory
```
Probably related to 6c68fbd4e1f8beac39cb1f499ff90c78256262d6.
This reverts commit 10addad6035034b2b78f3c74ef436cd7146d5231, reversing
changes made to 7786575c6c0e1b010d46ad00b14d0bb5bf08d7d2.
NixOS scripts should be kept in the NixOS source tree, not in
pkgs. Moving them around is just confusing and creates unnecessary
code/history churn.
Move all the nixos-* scripts from the nixos distribution as real
packages in the pkgs/ package set.
This allows non-nixos users to run the script as well. For example,
deploying a remote machine with:
nixos-rebuild --target-host root@hostname --build-host root@hostname
