ci: fixing the coverage report

[vincenzopalazzo]

I really do not have any clue why the ci is failing here https://github.com/lightningdevkit/rust-lightning/actions/runs/11683772490/job/32533657424?pr=3397 so trying out stuff

[vincenzopalazzo]

Ok probably I found the problem but I do not have any idea why it is happening

I was able to reproduce in a pure nix shell

nix-shell -p rustup --pure
cargo install cargo-llvm-cov
export RUSTFLAGS="-Clink-dead-code"
cargo llvm-cov --features rest-client,rpc-client,tokio,futures,serde --codecov --hide-instantiations --output-path=target/codecov.json

I do not know the history of the flag link-dead-code but around the internet, I read that the error can be a mismatch version between some dependencies, but not sure.

[TheBlueMatt]

Looks like just a bug in rustc, might as well just take this and see if codecov gets mad.