Allows installation via nix from the repository itself
on NixOS and other Linux distribution that have Nix (i.e. Archlinux/Debian).

```
$ nix-build
$ ./result/bin/kconfig-hardened-check
$ nix-env -f . -i
```

It also provides an development environment for `nix-shell` with setuptools and
python in path

```
$ nix-shell
```