feat(nix): improve flake.nix, follows nixpkgs

[bryango]

This patch tries to improve the flake.nix file and brings in nixpkgs updates. The changes may seem overwhelming at first, but they are organized into the following commits, each can be easily reviewed and approved (or rejected):

• Flatten the flake outputs for clarity, and use the updated flake entries instead of the deprecated ones:
  • packages.${system}.default instead of defaultPackage
  • overlays.default instead of overlay.
• Reuse nixpkgs definition for git-branchless, and supply local changes through overrides. This ensures that nixpkgs updates such as https://github.com/NixOS/nixpkgs/pull/289324 are automatically propagated to the package definition here. Some comments are added to direct people to contribute to nixpkgs for the package definition.
• Reuse git-branchless packaging for scm-diff-editor, since their package definitions are almost identical.
• Bump nixpkgs flake.lock. With the above changes in place this automatically pulls in the nixpkgs update https://github.com/NixOS/nixpkgs/pull/289324 (this is actually my PR, guilty haha).

Pinging @thoughtpolice and @hmenke if they are interested in a review!

[bryango]

Added some comments to direct people to contribute to nixpkgs for the package definition, and rebased.

[arxanas]

I can't read Nix so I'll just trust @hmenke and CI 😇