This allows users to put zig in their `mkShell` without ending up with a
ZIG_GLOBAL_CACHE_DIR value set that points back to a location that only
would exist in the sandbox.
By moving the zig setup hook to the zig derivation itself, we allow for
zig to splice correctly with `callPackage`, meaning that the correct zig
can be pulled in during builds when zig is in `nativeBuildInputs` (for
example).
This change retains the `zig.hook` attribute for backward compatibility
by just pointing to the zig derivation. This also removes
`zig_default_flags`, since now the setup hook is not a derivation that
can be overridden. Overriding the build flags can now be done by setting
`dontSetZigDefaultFlags = true`.
