Remove dependency on nixos-install

[ninegua]

The pitfall of using nixos-install in this script is that the hardware detection can be mis-guided (e.g. swap partition).
A better way is to follow the approach taken by nixos/modules/installer/cd-dvd/iso-image.nix, and use a pure Nix way to create a USB disk image.

Other hopeful benefits are

• A way to create a disk image (of only necessary size) that can be later written into USB drive.
• A way to create a disk image on a non-NixOS Linux or even OS X machine.

[ninegua]

Commit 8ab8bd7 implemented a nix based solution. The main caveat is that it had to use runInLinuxVM because there seems to be no easy way to use cryptsetup without first mounting the image file through a loopback device.

[ninegua]

It turns out that commit 8ab8bd7 wasn't safe, because nix would remember arguments in derivation files, and a passphrase would be in clear text in a nix-store! I need to figure out some other way to get around this problem.

Thanks to @nmattia for pointing out this glaring mistake! I've rolled back the master until a better solution surfaces.