Proper nix build

[sayanarijit]

In #84 (comment) @infinisil pointed out that:

... this is not a Nix build yet. How it is now, only the dependencies for the build are provided by Nix, the build itself is still done impurely and out of Nix's control. The only reason this was done like this originally was to keep the development workflow the same, but this has some disadvantages like the build being impure, closure size for the final product being too big, and too much complexity in the build process. This should be considered only a temporarily solution until Nix can be used fully (meaning a nix-build will build the whole project).

This issue is a reminder to make it a proper nix build. Using poetry2nix might be the way to go.