legacy executable should not require flakes or nix-command features

[water-sucks]

What Happened?

The flakes features are required in order to fetch and use the Nix package properly, due to its reliance on builtins.getFlake. This is due to the fact that package.nix relies on a fetchZigDeps function that comes from a flake.

Refactor the flake and package such that anyone can reliably use callPackage themselves on the Nix file without having to provide any parameters themselves.

Additionally, review usage of the nix command using the runCmd function and make sure that legacy commands use the nix-* equivalent forms of the commands, to avoid requiring the nix-command interface. It could be the case that there are no usages of nix-command in legacy-style code, in which case we can ignore this, but this is something that needs to be audited properly.

How To Reproduce

Attempt to use this flake in legacy configurations.

Expected Behavior

I expected to be able to use this executable without enabling the flakes and nix-command experimental features.

Features

(not relevant)

[water-sucks]

The rewrite will fix this due to fetchZigDeps not being present. A simple import of the default.nix should suffice after this.