nixos-rebuild: add --impure flag
nixos-rebuild: add --impure flag

There are two use case for this flag:

1. NixOS developer usually use a nixpkgs checkout for development.
Copying nixpkgs everytime when rebuilding NixOS is way to slow, even
with NVME disks.

2. Folks migrating from impure configuration in a sufficient complex
infrastructure need this flag to gradually migrate to NixOS flakes.

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