Add .cargo/config.toml

[racagogi]

Summary

rustc not use cmake LD_PATH see(#693 (comment))
thus, add config.toml to add link-args.
this solution work from stable to nightly rustc, rust-toolchain.toml can use stable channel.

Note

Checklist

• I have documented my changes properly to adequate places
• I have updated the docs/CHANGELOG.md

[jhhuh]

To me, putting -Clink-args=-L./target/release in .cargo/config.toml doesn't seem appropriate since it was a very hacky way of making it just work for the nix build. It's only for release build, which is the only target nix build should care for.

A better fix might be with build.sh or build.rs. I may look for it next week.