Add support for Nix develop through flakes

[pocalyps]

For the unfamiliar, flake.nix is vaguely similar to what a Dockerfile would be: It describes packages, dependencies, environment variables required to build the project.

This flake.nix should be all that's required for someone who wants to build this project under Nix. Although I haven't tested it myself, I believe the flake should support most system configuration as it doesn't have hard coded values, but if there was such a problem, I'd be more than happy to help and fix those as they arise.

Here's how someone would use this flake to build cosmic-greeter:

cd cosmic-greeter
nix develop
cargo build