Minor issues/suggestions on `flake.nix` and `.gitignore`

[LEXUGE]

ARTIQ Feature Request

Problem this request addresses

Several minor issues/suggestions on flake.nix and .gitignore

• [ ] add result produced by nix to .gitignore, and refactor .gitignore.
• [ ] format flake.nix by nixpkgs-fmt, a formatter used by nixpkgs.
• [ ] use flake-utils to avoid boilerplates like openocd-bscanspi-f and manually writing out pkgs-aarch64 and pkgs
• [ ] use pre-commit-hooks to automatically check formatting and lints with flake8, nixpkgs-fmt, and rustfmt before commit and with nix flake check.

Additional context

I locally tested flake8 lint, it seems the python codebase has a lot of lint errors.