How to switch compile options between stable/nightly in config.toml?

Having both stable and nightly Rust toolchain installed. And use config.toml rustflags to specify (-C... for stable) / (-Z... for nightly) compiling arguments; but after adding -Z flags the stable rust stop working due to unrecognized -Z options.

Q: How to config nightly features only when using nightly toolchain in config.toml file? Is there a way or conditional directive to help it ..