feat: stop using directories outside of OUT_DIR in build time

[nicolasauler]

Description

From cargo documentation (https://doc.rust-lang.org/cargo/reference/environment-variables.html):

In general, build scripts should not modify any files outside of OUT_DIR. It may seem fine on the first blush, but it does cause problems when you use such crate as a dependency, because there’s an implicit invariant that sources in .cargo/registry should be immutable. cargo won’t allow such scripts when packaging.

This indeed is happening when I try to run nix related shenanigans on Bipa Server, due to this dependency not following this guideline.