allow pure build of zig derivation

[oldaccountdeadname]

Just one commit, here's the message:

allow pure build of zig derivation

Prior to this commit, if nixpkgs was imported with a definite system, it
would *mostly* work as intended and not make impure calls to Nix's
builtins.CurrentSystem, except for here in zig_raw.nix, which doesn't
respect sources and pkgs. The solution is simple: just inherit sources
and pkgs when we make a versioned zig_raw in zig.nix.

I've tested that this successfully builds and that tests run, so I don't think this should cause any problems!

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This change is