Convert the Nix packaging to use flakes

[exarkun]

"Flakes" are a new feature of the Nix toolchain which provide a number of desirable features:

• built-in pinning & maintenance tools
• conventions and standards for providing multiple package types (and non-package outputs like NixOS modules)

And there are new tools that only work with flakes, such as a new automatic RPM / Deb / Docker image builder.