Get rid of direct gc dependency

[roberth]

We're currently generating code for Boehm GC / bdwgc.
Nix is technically not guaranteed to use that, so we shouldn't create that dependency.

Depends on NixOS/nix#10636

The goal is to remove this line, and still have a working evaluator:

https://github.com/nixops4/nixops4/blob/ef9d96e63c19164ea7637d1e3d0740d98451dae6/rust/nix-c-raw/include/nix-c-raw.h#L4