Use the rust-specific config when available

[Ericson2314]

Per the TODO, we could leverage Nixpkgs to do this better still, but it isn't terribly convenient at this time. I rather do it after the other cross PRs are merged.

[Ericson2314]

#203 and #212 need tests, but it's unfortunately a bit tricky to make some that are not slow.

[Ericson2314]

I'm not longer worried about testing this so much, because now that it uses a function and isn't conditional, the ball is more in Nixpkgs court (there is a single code path on the create2nix side).

So merely ensuring that existing functionality still works should be good.