refactor: nix flake for build and dev

[karlskewes]

The existing flake didn't build for me.
Refactored per: pop-os/cosmic-comp#2219 (comment)

I used the refactored flake for nix build and nix develop of #137.

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[mmstick]

Are these still needed for NixOS packaging?

[karlskewes]

Are these still needed for NixOS packaging?

Thanks for asking!

NixOS itself maintains its own packaging nix files like this: https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/co/cosmic-settings-daemon/package.nix

This PR and pop-os/cosmic-comp#2219 enable build and development of the Cosmic component in a nix "dev shell".
I used these to develop the linked pinned workspaces feature PR's.

Most of the Cosmic repo's have nix files already in various states of utility/working/broken.
So I figure the project could either abandon/remove them or I could go around and update them to be working and based on what NixOS do upstream which I figure is generally best practice.

Happy to mark ready/close/do the needful.