This allows adding pre-commit checks that can't be run in the sandbox,
such as `clippy` in Rust etc...
Not strictly necessary for the few checks that are enabled in the
current templates, but I like to have very similar setups across the
board.
I don't need to bootstrap `nix-direnv` for those projects.
Also remove the unused `watch_file flake/*.nix` directives, as they do
not exist in the templates.
