Updating hiem with older transitive dependency on nix

[froismo]

I'm new to rust, so maybe I'm mistaken, and if so, sorry for wasting your time.

Vector depends on (nix version 0.26.1](https://github.com/vectordotdev/vector/blob/master/Cargo.toml#L343), and a Vector-supported version of heim depends on ^0.23.0. When vector is built will the heim module have its own version of nix, or will it use the nix from the enclosing project?

Specifically, I'm hoping to get nix updated to remediate nix-rust/nix#1762 which is in nix versions above 0.25.0.