We're only using `eachDefaultSystem` from flake-utils so pulling it in is
a bit overkill; using the underlying nix-systems dependency directly brings
us closer to a "stock" flake format without adding unnecessary burden.
since this is purely a legacy concern (both default.nix and shell.nix),
we avoid to include the flake-compat as a flake input.
(even though unused inputs are lazily fetched - for style & clarity)