Improve the flake decalaration

[Sigmanificient]

Bring minor improvement the the flake.nix:

• Use devShell (singular) attribute instead of devShells (list)
• Resolve dev shell dependencies from the fzf-make package inputs.
• Added nixpkgs-fmt to ensure a proper flake formatting
• Use direct path from Cargo.toml
• Update package declaration to reflect nixpkgs

[Sigmanificient]

Well thanks you too, I've learn a lot about nix thanks to porting projects like yours