Function flux_sysroot_dir 

pub fn flux_sysroot_dir() -> PathBuf

Returns the path to the active Flux sysroot.

Both cargo-flux and flux call this at startup:

• cargo-flux uses it to locate flux-driver and sets RUSTC=<driver> before invoking cargo.
• flux uses it to locate flux-driver and passes -L <sysroot> so rustc can find the precompiled extern crates (flux_rs, flux_attrs).