fix: dead code linking

Previous commit 0d3abdc5 was incorrect (I got the correct result because of the dirty environment with exported RUSTFLAGS). I have found out that we can't pass link-dead-code option via the build script flags (there is no suitable parameter for it):

https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-arg

The only way to pass this codegen option to rustc seems to be env variable:

https://doc.rust-lang.org/rustc/codegen-options/index.html#link-dead-code

Close #229